Add PDF attachment and PDF/A conversion features

Introduced endpoints for embedding attachments in PDFs and converting
between standard PDFs and PDF/A formats. Added `PdfAttachmentController`
and `PdfConversionController` with multipart/form-data and Base64-based
support. Implemented commands, handlers, and validators for these
operations.

Extended `IPdfProcessor` with methods for adding attachments and
PDF/A conversion. Partially implemented functionality in
`DevExpressPdfProcessor`, including `ConvertFromPdfAAsync`.

Added `attachment.xml` and `withoutAttachment.pdf` as resources for
testing. Marked endpoints as `[Obsolete]` to indicate incomplete
implementation. Improved validation and error handling for commands.

DocumentOperator.API/Controllers/PdfAttachmentController.cs

@@ -1,3 +1,4 @@
+using DocumentOperator.Application.AddAttachments;
 using DocumentOperator.Application.CheckPdfAttachments.Queries;
 using DocumentOperator.Application.Common.DTOs;
 using DocumentOperator.Application.ExtractPdfAttachments;
@@ -157,6 +158,135 @@ public class PdfAttachmentController(IMediator mediator) : ControllerBase
         // Return ZIP file
         return File(zipBytes, "application/zip", "attachments.zip");
     }
+
+    /// <summary>
+    /// Embeds one or more files as attachments in a PDF document (supports PDF/A-3).
+    /// Supports multipart/form-data file upload.
+    /// </summary>
+    /// <param name="pdfFile">The PDF file to add attachments to</param>
+    /// <param name="attachmentFiles">Files to embed as attachments (one or more)</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>PDF with embedded attachments</returns>
+    /// <response code="200">Attachments added successfully - returns PDF</response>
+    /// <response code="400">Invalid input (file missing, not a PDF, or no attachments provided)</response>
+    /// <response code="500">Internal server error during PDF processing</response>
+    [Obsolete("This endpoint is not implemented yet.")]
+    [HttpPost("add")]
+    [Consumes("multipart/form-data")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> AddAttachmentsFromFile(
+        IFormFile pdfFile,
+        List<IFormFile> attachmentFiles,
+        CancellationToken cancellationToken)
+    {
+        if (pdfFile == null || pdfFile.Length == 0)
+        {
+            throw new BadRequestException("PDF file is required");
+        }
+
+        if (attachmentFiles == null || attachmentFiles.Count == 0)
+        {
+            throw new BadRequestException("At least one attachment file is required");
+        }
+
+        // Use IFormFile stream directly (no intermediate byte[] conversion)
+        using var pdfStream = pdfFile.OpenReadStream();
+
+        // Convert attachment files to AttachmentFile records
+        var attachments = new List<AttachmentFile>();
+        foreach (var file in attachmentFiles)
+        {
+            using var ms = new MemoryStream();
+            await file.CopyToAsync(ms, cancellationToken);
+            
+            attachments.Add(new AttachmentFile
+            {
+                FileName = file.FileName,
+                Content = ms.ToArray(),
+                MimeType = file.ContentType
+            });
+        }
+
+        // Send command to MediatR
+        var command = new AddAttachmentsCommand 
+        { 
+            PdfStream = pdfStream,
+            Attachments = attachments
+        };
+        byte[] resultPdf = await mediator.Send(command, cancellationToken);
+
+        // Return PDF with attachments
+        return File(resultPdf, "application/pdf", "with-attachments.pdf");
+    }
+
+    /// <summary>
+    /// Embeds one or more files as attachments in a PDF document (supports PDF/A-3).
+    /// Supports Base64-encoded PDF and attachments via JSON payload.
+    /// </summary>
+    /// <param name="request">Request containing Base64-encoded PDF and attachments</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>PDF with embedded attachments</returns>
+    /// <response code="200">Attachments added successfully - returns PDF</response>
+    /// <response code="400">Invalid input (Base64 format error, not a PDF, or no attachments provided)</response>
+    /// <response code="500">Internal server error during PDF processing</response>
+    [Obsolete("This endpoint is not implemented yet.")]
+    [HttpPost("add")]
+    [Consumes("application/json")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> AddAttachmentsFromBase64(
+        [FromBody] AddAttachmentsRequest request,
+        CancellationToken cancellationToken)
+    {
+        // Convert Base64 PDF to stream
+        byte[] pdfBytes;
+        try
+        {
+            pdfBytes = Convert.FromBase64String(request.Base64Pdf);
+        }
+        catch (FormatException ex)
+        {
+            throw new BadRequestException("Invalid Base64 PDF format: " + ex.Message);
+        }
+        
+        using var pdfStream = new MemoryStream(pdfBytes);
+
+        // Convert Base64 attachments to AttachmentFile records
+        var attachments = new List<AttachmentFile>();
+        foreach (var att in request.Attachments)
+        {
+            byte[] attBytes;
+            try
+            {
+                attBytes = Convert.FromBase64String(att.Base64Content);
+            }
+            catch (FormatException ex)
+            {
+                throw new BadRequestException($"Invalid Base64 format for attachment '{att.FileName}': " + ex.Message);
+            }
+
+            attachments.Add(new AttachmentFile
+            {
+                FileName = att.FileName,
+                Content = attBytes,
+                MimeType = att.MimeType
+            });
+        }
+
+        // Send command to MediatR
+        var command = new AddAttachmentsCommand 
+        { 
+            PdfStream = pdfStream,
+            Attachments = attachments
+        };
+        byte[] resultPdf = await mediator.Send(command, cancellationToken);
+
+        // Return PDF with attachments
+        return File(resultPdf, "application/pdf", "with-attachments.pdf");
+    }
 }
 
 /// <summary>
@@ -182,3 +312,44 @@ public record ExtractPdfAttachmentsRequest
     /// <example>JVBERi0xLjQKJeLjz9MKMSAwIG9iago8PC9UeXBlL0NhdGFsb2c...</example>
     public required string Base64Pdf { get; init; }
 }
+
+/// <summary>
+/// Request DTO for Base64-encoded PDF with attachments to add
+/// </summary>
+public record AddAttachmentsRequest
+{
+    /// <summary>
+    /// PDF document encoded as Base64 string
+    /// </summary>
+    /// <example>JVBERi0xLjQKJeLjz9MKMSAwIG9iago8PC9UeXBlL0NhdGFsb2c...</example>
+    public required string Base64Pdf { get; init; }
+
+    /// <summary>
+    /// List of attachments to embed
+    /// </summary>
+    public required List<AttachmentRequestDto> Attachments { get; init; }
+}
+
+/// <summary>
+/// DTO for attachment file in request
+/// </summary>
+public record AttachmentRequestDto
+{
+    /// <summary>
+    /// File name (e.g., "invoice.xml", "document.pdf")
+    /// </summary>
+    /// <example>factur-x.xml</example>
+    public required string FileName { get; init; }
+
+    /// <summary>
+    /// File content encoded as Base64 string
+    /// </summary>
+    /// <example>PD94bWwgdmVyc2lvbj0iMS4wIj8+...</example>
+    public required string Base64Content { get; init; }
+
+    /// <summary>
+    /// MIME type (optional, e.g., "application/xml")
+    /// </summary>
+    /// <example>application/xml</example>
+    public string? MimeType { get; init; }
+}

DocumentOperator.API/Controllers/PdfConversionController.cs

@@ -0,0 +1,211 @@
+using DocumentOperator.Application.ConvertFromPdfA;
+using DocumentOperator.Application.ConvertToPdfA;
+using DocumentOperator.Domain.Common.Exceptions;
+using MediatR;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+
+namespace DocumentOperator.API.Controllers;
+
+/// <summary>
+/// Controller for PDF conversion operations (PDF ? PDF/A)
+/// </summary>
+[ApiController]
+[Route("api/pdf/conversion")]
+[Obsolete("This endpoint is not implemented yet.")]
+public class PdfConversionController(IMediator mediator) : ControllerBase
+{
+    /// <summary>
+    /// Converts a standard PDF to PDF/A format.
+    /// Supports multipart/form-data file upload.
+    /// </summary>
+    /// <param name="file">The PDF file to convert</param>
+    /// <param name="pdfALevel">Target PDF/A level (e.g., "PDF/A-1b", "PDF/A-2b", "PDF/A-3b")</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>PDF/A compliant document</returns>
+    /// <response code="200">PDF converted to PDF/A successfully</response>
+    /// <response code="400">Invalid input (file missing, not a PDF, or invalid PDF/A level)</response>
+    /// <response code="500">Internal server error during PDF processing</response>
+    [Obsolete("This endpoint is not implemented yet.")]
+    [HttpPost("to-pdfa")]
+    [Consumes("multipart/form-data")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> ConvertToPdfAFromFile(
+        IFormFile file,
+        [FromQuery] string pdfALevel = "PDF/A-3b",
+        CancellationToken cancellationToken = default)
+    {
+        if (file == null || file.Length == 0)
+        {
+            throw new BadRequestException("PDF file is required");
+        }
+
+        // Use IFormFile stream directly (no intermediate byte[] conversion)
+        using var pdfStream = file.OpenReadStream();
+
+        // Send command to MediatR
+        var command = new ConvertToPdfACommand 
+        { 
+            PdfStream = pdfStream,
+            PdfALevel = pdfALevel
+        };
+        byte[] resultPdf = await mediator.Send(command, cancellationToken);
+
+        // Return PDF/A file
+        return File(resultPdf, "application/pdf", "converted-pdfa.pdf");
+    }
+
+    /// <summary>
+    /// Converts a standard PDF to PDF/A format.
+    /// Supports Base64-encoded PDF via JSON payload.
+    /// </summary>
+    /// <param name="request">Request containing Base64-encoded PDF and PDF/A level</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>PDF/A compliant document</returns>
+    /// <response code="200">PDF converted to PDF/A successfully</response>
+    /// <response code="400">Invalid input (Base64 format error, not a PDF, or invalid PDF/A level)</response>
+    /// <response code="500">Internal server error during PDF processing</response>
+    [Obsolete("This endpoint is not implemented yet.")]
+    [HttpPost("to-pdfa")]
+    [Consumes("application/json")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> ConvertToPdfAFromBase64(
+        [FromBody] ConvertToPdfARequest request,
+        CancellationToken cancellationToken = default)
+    {
+        // Convert Base64 PDF to stream
+        byte[] pdfBytes;
+        try
+        {
+            pdfBytes = Convert.FromBase64String(request.Base64Pdf);
+        }
+        catch (FormatException ex)
+        {
+            throw new BadRequestException("Invalid Base64 PDF format: " + ex.Message);
+        }
+        
+        using var pdfStream = new MemoryStream(pdfBytes);
+
+        // Send command to MediatR
+        var command = new ConvertToPdfACommand 
+        { 
+            PdfStream = pdfStream,
+            PdfALevel = request.PdfALevel ?? "PDF/A-3b"
+        };
+        byte[] resultPdf = await mediator.Send(command, cancellationToken);
+
+        // Return PDF/A file
+        return File(resultPdf, "application/pdf", "converted-pdfa.pdf");
+    }
+
+    /// <summary>
+    /// Converts a PDF/A document to a standard PDF (removes PDF/A restrictions).
+    /// Supports multipart/form-data file upload.
+    /// </summary>
+    /// <param name="file">The PDF/A file to convert</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Standard PDF document</returns>
+    /// <response code="200">PDF/A converted to standard PDF successfully</response>
+    /// <response code="400">Invalid input (file missing, not a PDF)</response>
+    /// <response code="500">Internal server error during PDF processing</response>
+    [Obsolete("This endpoint is not implemented yet.")]
+    [HttpPost("from-pdfa")]
+    [Consumes("multipart/form-data")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> ConvertFromPdfAFromFile(
+        IFormFile file,
+        CancellationToken cancellationToken = default)
+    {
+        if (file == null || file.Length == 0)
+        {
+            throw new BadRequestException("PDF/A file is required");
+        }
+
+        // Use IFormFile stream directly (no intermediate byte[] conversion)
+        using var pdfStream = file.OpenReadStream();
+
+        // Send command to MediatR
+        var command = new ConvertFromPdfACommand { PdfStream = pdfStream };
+        byte[] resultPdf = await mediator.Send(command, cancellationToken);
+
+        // Return standard PDF file
+        return File(resultPdf, "application/pdf", "converted-pdf.pdf");
+    }
+
+    /// <summary>
+    /// Converts a PDF/A document to a standard PDF (removes PDF/A restrictions).
+    /// Supports Base64-encoded PDF via JSON payload.
+    /// </summary>
+    /// <param name="request">Request containing Base64-encoded PDF/A</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Standard PDF document</returns>
+    /// <response code="200">PDF/A converted to standard PDF successfully</response>
+    /// <response code="400">Invalid input (Base64 format error, not a PDF)</response>
+    /// <response code="500">Internal server error during PDF processing</response>
+    [Obsolete("This endpoint is not implemented yet.")]
+    [HttpPost("from-pdfa")]
+    [Consumes("application/json")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> ConvertFromPdfAFromBase64(
+        [FromBody] ConvertFromPdfARequest request,
+        CancellationToken cancellationToken = default)
+    {
+        // Convert Base64 PDF to stream
+        byte[] pdfBytes;
+        try
+        {
+            pdfBytes = Convert.FromBase64String(request.Base64Pdf);
+        }
+        catch (FormatException ex)
+        {
+            throw new BadRequestException("Invalid Base64 PDF format: " + ex.Message);
+        }
+        
+        using var pdfStream = new MemoryStream(pdfBytes);
+
+        // Send command to MediatR
+        var command = new ConvertFromPdfACommand { PdfStream = pdfStream };
+        byte[] resultPdf = await mediator.Send(command, cancellationToken);
+
+        // Return standard PDF file
+        return File(resultPdf, "application/pdf", "converted-pdf.pdf");
+    }
+}
+
+/// <summary>
+/// Request DTO for converting PDF to PDF/A
+/// </summary>
+public record ConvertToPdfARequest
+{
+    /// <summary>
+    /// PDF document encoded as Base64 string
+    /// </summary>
+    /// <example>JVBERi0xLjQKJeLjz9MKMSAwIG9iago8PC9UeXBlL0NhdGFsb2c...</example>
+    public required string Base64Pdf { get; init; }
+
+    /// <summary>
+    /// Target PDF/A level (default: "PDF/A-3b")
+    /// </summary>
+    /// <example>PDF/A-3b</example>
+    public string? PdfALevel { get; init; }
+}
+
+/// <summary>
+/// Request DTO for converting PDF/A to PDF
+/// </summary>
+public record ConvertFromPdfARequest
+{
+    /// <summary>
+    /// PDF/A document encoded as Base64 string
+    /// </summary>
+    /// <example>JVBERi0xLjQKJeLjz9MKMSAwIG9iago8PC9UeXBlL0NhdGFsb2c...</example>
+    public required string Base64Pdf { get; init; }
+}

DocumentOperator.Application/AddAttachments/AddAttachmentsCommand.cs

@@ -0,0 +1,90 @@
+using DocumentOperator.Application.Common.Interfaces;
+using FluentValidation;
+using MediatR;
+
+namespace DocumentOperator.Application.AddAttachments;
+
+/// <summary>
+/// Command to add one or more attachments to a PDF document (supports PDF/A-3)
+/// </summary>
+public record AddAttachmentsCommand : IRequest<byte[]>
+{
+    /// <summary>
+    /// PDF document stream. Must be positioned at the beginning (Position = 0).
+    /// </summary>
+    public required Stream PdfStream { get; init; }
+
+    /// <summary>
+    /// List of attachments to embed (filename, content, optional MIME type)
+    /// </summary>
+    public required IReadOnlyList<AttachmentFile> Attachments { get; init; }
+}
+
+/// <summary>
+/// Represents a file to be attached to a PDF
+/// </summary>
+public record AttachmentFile
+{
+    /// <summary>
+    /// File name (e.g., "invoice.xml", "document.pdf")
+    /// </summary>
+    public required string FileName { get; init; }
+
+    /// <summary>
+    /// File content as byte array
+    /// </summary>
+    public required byte[] Content { get; init; }
+
+    /// <summary>
+    /// MIME type (optional, e.g., "application/xml", "application/pdf")
+    /// If not provided, will be inferred from file extension
+    /// </summary>
+    public string? MimeType { get; init; }
+}
+
+/// <summary>
+/// Handler for AddAttachmentsCommand
+/// </summary>
+public class AddAttachmentsCommandHandler(IPdfProcessor pdfProcessor) 
+    : IRequestHandler<AddAttachmentsCommand, byte[]>
+{
+    public async Task<byte[]> Handle(AddAttachmentsCommand request, CancellationToken cancellationToken)
+    {
+        // Convert to tuple list for IPdfProcessor
+        var attachmentTuples = request.Attachments
+            .Select(a => (a.FileName, a.Content, a.MimeType))
+            .ToList();
+
+        return await pdfProcessor.AddAttachmentsAsync(request.PdfStream, attachmentTuples);
+    }
+}
+
+/// <summary>
+/// Validator for AddAttachmentsCommand
+/// </summary>
+public class AddAttachmentsCommandValidator : AbstractValidator<AddAttachmentsCommand>
+{
+    public AddAttachmentsCommandValidator()
+    {
+        RuleFor(x => x.PdfStream)
+            .NotNull()
+            .WithMessage("PDF stream is required");
+
+        RuleFor(x => x.Attachments)
+            .NotNull()
+            .NotEmpty()
+            .WithMessage("At least one attachment is required");
+
+        RuleForEach(x => x.Attachments).ChildRules(attachment =>
+        {
+            attachment.RuleFor(a => a.FileName)
+                .NotEmpty()
+                .WithMessage("Attachment file name is required");
+
+            attachment.RuleFor(a => a.Content)
+                .NotNull()
+                .NotEmpty()
+                .WithMessage("Attachment content is required");
+        });
+    }
+}

DocumentOperator.Application/Common/Interfaces/IPdfProcessor.cs

@@ -145,4 +145,47 @@ public interface IPdfProcessor
         Domain.Models.ValueObjects.StampPlacement placement = Domain.Models.ValueObjects.StampPlacement.Foreground,
         byte[]? imageBytes = null,
         Domain.Models.ValueObjects.PredefinedStampType? predefinedType = null);
+
+    /// <summary>
+    /// Embeds one or more files as attachments in a PDF document (supports PDF/A-3).
+    /// </summary>
+    /// <param name="pdfStream">
+    /// PDF document stream. Must be readable and positioned at the beginning (Position = 0).
+    /// Non-seekable streams are supported. Caller is responsible for disposal.
+    /// </param>
+    /// <param name="attachments">List of files to embed (filename, content, optional MIME type)</param>
+    /// <returns>PDF with embedded attachments as byte array</returns>
+    /// <exception cref="Domain.Common.Exceptions.BadRequestException">
+    /// Thrown when stream is empty/invalid, or attachments list is empty
+    /// </exception>
+    Task<byte[]> AddAttachmentsAsync(
+        Stream pdfStream, 
+        IReadOnlyList<(string FileName, byte[] Content, string? MimeType)> attachments);
+
+    /// <summary>
+    /// Converts a standard PDF to PDF/A format.
+    /// </summary>
+    /// <param name="pdfStream">
+    /// PDF document stream. Must be readable and positioned at the beginning (Position = 0).
+    /// Non-seekable streams are supported. Caller is responsible for disposal.
+    /// </param>
+    /// <param name="pdfALevel">Target PDF/A level (e.g., "PDF/A-1b", "PDF/A-2b", "PDF/A-3b")</param>
+    /// <returns>PDF/A compliant document as byte array</returns>
+    /// <exception cref="Domain.Common.Exceptions.BadRequestException">
+    /// Thrown when stream is empty/invalid or PDF/A level is unsupported
+    /// </exception>
+    Task<byte[]> ConvertToPdfAAsync(Stream pdfStream, string pdfALevel);
+
+    /// <summary>
+    /// Converts a PDF/A document to a standard PDF (removes PDF/A restrictions).
+    /// </summary>
+    /// <param name="pdfStream">
+    /// PDF/A document stream. Must be readable and positioned at the beginning (Position = 0).
+    /// Non-seekable streams are supported. Caller is responsible for disposal.
+    /// </param>
+    /// <returns>Standard PDF document as byte array</returns>
+    /// <exception cref="Domain.Common.Exceptions.BadRequestException">
+    /// Thrown when stream is empty/invalid
+    /// </exception>
+    Task<byte[]> ConvertFromPdfAAsync(Stream pdfStream);
 }

DocumentOperator.Application/ConvertFromPdfA/ConvertFromPdfACommand.cs

@@ -0,0 +1,41 @@
+using DocumentOperator.Application.Common.Interfaces;
+using FluentValidation;
+using MediatR;
+
+namespace DocumentOperator.Application.ConvertFromPdfA;
+
+/// <summary>
+/// Command to convert a PDF/A document to a standard PDF (removes PDF/A restrictions)
+/// </summary>
+public record ConvertFromPdfACommand : IRequest<byte[]>
+{
+    /// <summary>
+    /// PDF/A document stream. Must be positioned at the beginning (Position = 0).
+    /// </summary>
+    public required Stream PdfStream { get; init; }
+}
+
+/// <summary>
+/// Handler for ConvertFromPdfACommand
+/// </summary>
+public class ConvertFromPdfACommandHandler(IPdfProcessor pdfProcessor) 
+    : IRequestHandler<ConvertFromPdfACommand, byte[]>
+{
+    public async Task<byte[]> Handle(ConvertFromPdfACommand request, CancellationToken cancellationToken)
+    {
+        return await pdfProcessor.ConvertFromPdfAAsync(request.PdfStream);
+    }
+}
+
+/// <summary>
+/// Validator for ConvertFromPdfACommand
+/// </summary>
+public class ConvertFromPdfACommandValidator : AbstractValidator<ConvertFromPdfACommand>
+{
+    public ConvertFromPdfACommandValidator()
+    {
+        RuleFor(x => x.PdfStream)
+            .NotNull()
+            .WithMessage("PDF stream is required");
+    }
+}

DocumentOperator.Application/ConvertToPdfA/ConvertToPdfACommand.cs

@@ -0,0 +1,59 @@
+using DocumentOperator.Application.Common.Interfaces;
+using FluentValidation;
+using MediatR;
+
+namespace DocumentOperator.Application.ConvertToPdfA;
+
+/// <summary>
+/// Command to convert a standard PDF to PDF/A format
+/// </summary>
+public record ConvertToPdfACommand : IRequest<byte[]>
+{
+    /// <summary>
+    /// PDF document stream. Must be positioned at the beginning (Position = 0).
+    /// </summary>
+    public required Stream PdfStream { get; init; }
+
+    /// <summary>
+    /// Target PDF/A level (e.g., "PDF/A-1b", "PDF/A-2b", "PDF/A-3b")
+    /// </summary>
+    public required string PdfALevel { get; init; }
+}
+
+/// <summary>
+/// Handler for ConvertToPdfACommand
+/// </summary>
+public class ConvertToPdfACommandHandler(IPdfProcessor pdfProcessor) 
+    : IRequestHandler<ConvertToPdfACommand, byte[]>
+{
+    public async Task<byte[]> Handle(ConvertToPdfACommand request, CancellationToken cancellationToken)
+    {
+        return await pdfProcessor.ConvertToPdfAAsync(request.PdfStream, request.PdfALevel);
+    }
+}
+
+/// <summary>
+/// Validator for ConvertToPdfACommand
+/// </summary>
+public class ConvertToPdfACommandValidator : AbstractValidator<ConvertToPdfACommand>
+{
+    private static readonly string[] ValidPdfALevels = 
+    {
+        "PDF/A-1b", "PDF/A-1a",
+        "PDF/A-2b", "PDF/A-2u", "PDF/A-2a",
+        "PDF/A-3b", "PDF/A-3u", "PDF/A-3a"
+    };
+
+    public ConvertToPdfACommandValidator()
+    {
+        RuleFor(x => x.PdfStream)
+            .NotNull()
+            .WithMessage("PDF stream is required");
+
+        RuleFor(x => x.PdfALevel)
+            .NotEmpty()
+            .WithMessage("PDF/A level is required")
+            .Must(level => ValidPdfALevels.Contains(level, StringComparer.OrdinalIgnoreCase))
+            .WithMessage($"Invalid PDF/A level. Valid values: {string.Join(", ", ValidPdfALevels)}");
+    }
+}

DocumentOperator.Infrastructure/Services/PdfProcessing/DevExpressPdfProcessor.cs

@@ -1093,4 +1093,113 @@ public class DevExpressPdfProcessor : IPdfProcessor
     }
 
     #endregion
+
+    #region Add Attachments (Phase 2)
+
+    /// <summary>
+    /// Embeds one or more files as attachments in a PDF document (supports PDF/A-3).
+    /// </summary>
+    public async Task<byte[]> AddAttachmentsAsync(
+        Stream pdfStream,
+        IReadOnlyList<(string FileName, byte[] Content, string? MimeType)> attachments)
+    {
+        // 1. Validate input
+        ArgumentNullException.ThrowIfNull(pdfStream, nameof(pdfStream));
+        ArgumentNullException.ThrowIfNull(attachments, nameof(attachments));
+
+        if (pdfStream.Length == 0)
+            throw new BadRequestException("PDF stream cannot be empty");
+
+        if (pdfStream.Position != 0)
+            throw new BadRequestException("PDF stream must be at position 0");
+
+        if (attachments.Count == 0)
+            throw new BadRequestException("At least one attachment is required");
+
+        // TODO: Implement AddFileAttachment using DevExpress.Pdf low-level API
+        // Current limitation: DevExpress.Pdf.PdfDocumentProcessor doesn't directly support adding attachments
+        // Workaround options:
+        // 1. Use PdfDocumentProcessor.Document to manipulate PDF structure directly (advanced)
+        // 2. Use third-party library for this specific operation
+        // 3. Wait for DevExpress API update
+
+        throw new NotImplementedException(
+            "Add attachments feature is not yet implemented. " +
+            "DevExpress.Pdf high-level API doesn't directly support adding file attachments. " +
+            "This requires low-level PDF structure manipulation.");
+    }
+
+    private string InferMimeType(string fileName)
+    {
+        string extension = Path.GetExtension(fileName).ToLowerInvariant();
+        return extension switch
+        {
+            ".xml" => "application/xml",
+            ".pdf" => "application/pdf",
+            ".json" => "application/json",
+            ".txt" => "text/plain",
+            ".jpg" or ".jpeg" => "image/jpeg",
+            ".png" => "image/png",
+            _ => "application/octet-stream"
+        };
+    }
+
+    #endregion
+
+    #region PDF Conversion (Phase 3)
+
+    /// <summary>
+    /// Converts a standard PDF to PDF/A format.
+    /// </summary>
+    public async Task<byte[]> ConvertToPdfAAsync(Stream pdfStream, string pdfALevel)
+    {
+        // 1. Validate input
+        ArgumentNullException.ThrowIfNull(pdfStream, nameof(pdfStream));
+
+        if (pdfStream.Length == 0)
+            throw new BadRequestException("PDF stream cannot be empty");
+
+        if (pdfStream.Position != 0)
+            throw new BadRequestException("PDF stream must be at position 0");
+
+        if (string.IsNullOrWhiteSpace(pdfALevel))
+            throw new BadRequestException("PDF/A level is required");
+
+        // TODO: Implement PDF to PDF/A conversion using DevExpress
+        // Current limitation: DevExpress.Pdf.PdfDocumentProcessor doesn't directly support PDF/A conversion
+        // Requires using specialized PDF/A conversion libraries or low-level PDF manipulation
+
+        throw new NotImplementedException(
+            $"PDF to PDF/A conversion ({pdfALevel}) is not yet implemented. " +
+            "DevExpress.Pdf high-level API doesn't directly support PDF/A conversion. " +
+            "This requires specialized PDF/A conversion logic.");
+    }
+
+    /// <summary>
+    /// Converts a PDF/A document to a standard PDF (removes PDF/A restrictions).
+    /// </summary>
+    public async Task<byte[]> ConvertFromPdfAAsync(Stream pdfStream)
+    {
+        // 1. Validate input
+        ArgumentNullException.ThrowIfNull(pdfStream, nameof(pdfStream));
+
+        if (pdfStream.Length == 0)
+            throw new BadRequestException("PDF stream cannot be empty");
+
+        if (pdfStream.Position != 0)
+            throw new BadRequestException("PDF stream must be at position 0");
+
+        // 2. Load PDF/A
+        using var processor = new PdfDocumentProcessor();
+        processor.LoadDocument(pdfStream);
+
+        // 3. Save as standard PDF
+        // DevExpress SaveDocument without special options creates standard PDF
+        using var outputStream = new MemoryStream();
+        processor.SaveDocument(outputStream);
+        
+        return await Task.FromResult(outputStream.ToArray());
+    }
+
+    #endregion
 }

DocumentOperator.Tests/TestData/Pdfs/attachment.xml

@@ -0,0 +1,152 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<rsm:CrossIndustryInvoice xmlns:rsm="urn:un:unece:uncefact:data:standard:CrossIndustryInvoice:100" xmlns:ram="urn:un:unece:uncefact:data:standard:ReusableAggregateBusinessInformationEntity:100" xmlns:udt="urn:un:unece:uncefact:data:standard:UnqualifiedDataType:100" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:un:unece:uncefact:data:standard:CrossIndustryInvoice:100 ../../../schemas/UN_CEFACT/CrossIndustryInvoice_100pD16B.xsd">
+  <rsm:ExchangedDocumentContext>
+    <ram:GuidelineSpecifiedDocumentContextParameter>
+      <ram:ID>urn:cen.eu:en16931:2017</ram:ID>
+    </ram:GuidelineSpecifiedDocumentContextParameter>
+  </rsm:ExchangedDocumentContext>
+  <rsm:ExchangedDocument>
+    <ram:ID>2021_10</ram:ID>
+    <ram:TypeCode>380</ram:TypeCode>
+    <ram:IssueDateTime>
+      <udt:DateTimeString format="102">20210924</udt:DateTimeString>
+    </ram:IssueDateTime>
+  </rsm:ExchangedDocument>
+  <rsm:SupplyChainTradeTransaction>
+    <ram:IncludedSupplyChainTradeLineItem>
+      <ram:AssociatedDocumentLineDocument>
+        <ram:LineID>1</ram:LineID>
+      </ram:AssociatedDocumentLineDocument>
+      <ram:SpecifiedTradeProduct>
+        <ram:Name>Project management</ram:Name>
+        <ram:Description/>
+      </ram:SpecifiedTradeProduct>
+      <ram:SpecifiedLineTradeAgreement>
+        <ram:NetPriceProductTradePrice>
+          <ram:ChargeAmount>500.000000</ram:ChargeAmount>
+        </ram:NetPriceProductTradePrice>
+      </ram:SpecifiedLineTradeAgreement>
+      <ram:SpecifiedLineTradeDelivery>
+        <ram:BilledQuantity unitCode="C62">2.00</ram:BilledQuantity>
+      </ram:SpecifiedLineTradeDelivery>
+      <ram:SpecifiedLineTradeSettlement>
+        <ram:ApplicableTradeTax>
+          <ram:TypeCode>VAT</ram:TypeCode>
+          <ram:CategoryCode>S</ram:CategoryCode>
+          <ram:RateApplicablePercent>19.00</ram:RateApplicablePercent>
+        </ram:ApplicableTradeTax>
+        <ram:SpecifiedTradeSettlementLineMonetarySummation>
+          <ram:LineTotalAmount>1000.00</ram:LineTotalAmount>
+        </ram:SpecifiedTradeSettlementLineMonetarySummation>
+      </ram:SpecifiedLineTradeSettlement>
+    </ram:IncludedSupplyChainTradeLineItem>
+    <ram:IncludedSupplyChainTradeLineItem>
+      <ram:AssociatedDocumentLineDocument>
+        <ram:LineID>2</ram:LineID>
+      </ram:AssociatedDocumentLineDocument>
+      <ram:SpecifiedTradeProduct>
+        <ram:Name>Consulting</ram:Name>
+        <ram:Description/>
+      </ram:SpecifiedTradeProduct>
+      <ram:SpecifiedLineTradeAgreement>
+        <ram:NetPriceProductTradePrice>
+          <ram:ChargeAmount>40.000000</ram:ChargeAmount>
+        </ram:NetPriceProductTradePrice>
+      </ram:SpecifiedLineTradeAgreement>
+      <ram:SpecifiedLineTradeDelivery>
+        <ram:BilledQuantity unitCode="C62">5.00</ram:BilledQuantity>
+      </ram:SpecifiedLineTradeDelivery>
+      <ram:SpecifiedLineTradeSettlement>
+        <ram:ApplicableTradeTax>
+          <ram:TypeCode>VAT</ram:TypeCode>
+          <ram:CategoryCode>S</ram:CategoryCode>
+          <ram:RateApplicablePercent>19.00</ram:RateApplicablePercent>
+        </ram:ApplicableTradeTax>
+        <ram:SpecifiedTradeSettlementLineMonetarySummation>
+          <ram:LineTotalAmount>200.00</ram:LineTotalAmount>
+        </ram:SpecifiedTradeSettlementLineMonetarySummation>
+      </ram:SpecifiedLineTradeSettlement>
+    </ram:IncludedSupplyChainTradeLineItem>
+    <ram:ApplicableHeaderTradeAgreement>
+      <ram:BuyerReference>139877</ram:BuyerReference>
+      <ram:SellerTradeParty>
+        <ram:Name>Webware Internet Solutions GmbH</ram:Name>
+        <ram:SpecifiedLegalOrganization>
+          <ram:ID>HRB 15635</ram:ID>
+        </ram:SpecifiedLegalOrganization>
+        <ram:DefinedTradeContact>
+          <ram:PersonName>John Doe</ram:PersonName>
+          <ram:TelephoneUniversalCommunication>
+            <ram:CompleteNumber>+49(0)561-560123456</ram:CompleteNumber>
+          </ram:TelephoneUniversalCommunication>
+          <ram:EmailURIUniversalCommunication>
+            <ram:URIID>johndoe@webware24.de</ram:URIID>
+          </ram:EmailURIUniversalCommunication>
+        </ram:DefinedTradeContact>
+        <ram:PostalTradeAddress>
+          <ram:PostcodeCode>34130</ram:PostcodeCode>
+          <ram:LineOne>Teichstr. 14-16</ram:LineOne>
+          <ram:CityName>Kassel</ram:CityName>
+          <ram:CountryID>DE</ram:CountryID>
+        </ram:PostalTradeAddress>
+        <ram:URIUniversalCommunication>
+          <ram:URIID schemeID="9930">DE279247134</ram:URIID>
+        </ram:URIUniversalCommunication>
+        <ram:SpecifiedTaxRegistration>
+          <ram:ID schemeID="FC">262/481/0918</ram:ID>
+        </ram:SpecifiedTaxRegistration>
+        <ram:SpecifiedTaxRegistration>
+          <ram:ID schemeID="VA">DE279247134</ram:ID>
+        </ram:SpecifiedTaxRegistration>
+      </ram:SellerTradeParty>
+      <ram:BuyerTradeParty>
+        <ram:Name>Agoratech</ram:Name>
+        <ram:PostalTradeAddress>
+          <ram:PostcodeCode>34130</ram:PostcodeCode>
+          <ram:LineOne>Teichstr. 14-16</ram:LineOne>
+          <ram:CityName>Kassel</ram:CityName>
+          <ram:CountryID>DE</ram:CountryID>
+        </ram:PostalTradeAddress>
+        <ram:URIUniversalCommunication>
+          <ram:URIID schemeID="9930">DE319642369</ram:URIID>
+        </ram:URIUniversalCommunication>
+      </ram:BuyerTradeParty>
+    </ram:ApplicableHeaderTradeAgreement>
+    <ram:ApplicableHeaderTradeDelivery>
+      <ram:ActualDeliverySupplyChainEvent>
+        <ram:OccurrenceDateTime>
+          <udt:DateTimeString format="102">20211101</udt:DateTimeString>
+        </ram:OccurrenceDateTime>
+      </ram:ActualDeliverySupplyChainEvent>
+    </ram:ApplicableHeaderTradeDelivery>
+    <ram:ApplicableHeaderTradeSettlement>
+      <ram:InvoiceCurrencyCode>EUR</ram:InvoiceCurrencyCode>
+      <ram:SpecifiedTradeSettlementPaymentMeans>
+        <ram:TypeCode>42</ram:TypeCode>
+        <ram:PayeePartyCreditorFinancialAccount>
+          <ram:IBANID/>
+        </ram:PayeePartyCreditorFinancialAccount>
+        <ram:PayeeSpecifiedCreditorFinancialInstitution>
+          <ram:BICID/>
+        </ram:PayeeSpecifiedCreditorFinancialInstitution>
+      </ram:SpecifiedTradeSettlementPaymentMeans>
+      <ram:ApplicableTradeTax>
+        <ram:CalculatedAmount>228</ram:CalculatedAmount>
+        <ram:TypeCode>VAT</ram:TypeCode>
+        <ram:BasisAmount>1200</ram:BasisAmount>
+        <ram:CategoryCode>S</ram:CategoryCode>
+        <ram:RateApplicablePercent>19.00</ram:RateApplicablePercent>
+      </ram:ApplicableTradeTax>
+      <ram:SpecifiedTradeSettlementHeaderMonetarySummation>
+        <ram:LineTotalAmount>1200</ram:LineTotalAmount>
+        <ram:ChargeTotalAmount>0</ram:ChargeTotalAmount>
+        <ram:AllowanceTotalAmount>0</ram:AllowanceTotalAmount>
+        <ram:TaxBasisTotalAmount>1200.00</ram:TaxBasisTotalAmount>
+        <ram:TaxTotalAmount currencyID="EUR">228.00</ram:TaxTotalAmount>
+        <ram:GrandTotalAmount>1428.00</ram:GrandTotalAmount>
+        <ram:TotalPrepaidAmount>0.00</ram:TotalPrepaidAmount>
+        <ram:DuePayableAmount>1428.00</ram:DuePayableAmount>
+      </ram:SpecifiedTradeSettlementHeaderMonetarySummation>
+    </ram:ApplicableHeaderTradeSettlement>
+  </rsm:SupplyChainTradeTransaction>
+</rsm:CrossIndustryInvoice>