diff --git a/DocumentOperator.API/Controllers/PdfOperationsController.cs b/DocumentOperator.API/Controllers/PdfOperationsController.cs
index 663572a..40940c4 100644
--- a/DocumentOperator.API/Controllers/PdfOperationsController.cs
+++ b/DocumentOperator.API/Controllers/PdfOperationsController.cs
@@ -1,5 +1,7 @@
+using DocumentOperator.Application.AddAnnotation;
 using DocumentOperator.Application.MergePdfs;
 using DocumentOperator.Domain.Common.Exceptions;
+using DocumentOperator.Domain.Models.ValueObjects;
 using MediatR;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc;
@@ -23,7 +25,7 @@ public class PdfOperationsController(IMediator mediator) : ControllerBase
     /// <response code="200">PDFs merged successfully - returns merged PDF</response>
     /// <response code="400">Invalid input (fewer than 2 files, corrupted PDF)</response>
     /// <response code="500">Internal server error during PDF processing</response>
-    [HttpPost("merge")]
+    [HttpPost("merge", Name = "MergeFromFiles")]
     [Consumes("multipart/form-data")]
     [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
     [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
@@ -58,17 +60,17 @@ public class PdfOperationsController(IMediator mediator) : ControllerBase
     /// <response code="200">PDFs merged successfully - returns merged PDF</response>
     /// <response code="400">Invalid input (Base64 format error, fewer than 2 files, corrupted PDF, invalid page range)</response>
     /// <response code="500">Internal server error during PDF processing</response>
-    [HttpPost("merge")]
+    [HttpPost("merge", Name = "MergeFromBase64")]
     [Consumes("application/json")]
     [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
     [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
     [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
     public async Task<IActionResult> MergeFromBase64(
-        [FromBody] MergePdfsRequest request,
+        [FromBody] MergePdfsBase64Request request,
         CancellationToken cancellationToken)
     {
         // Convert Base64[] to MemoryStream[]
-        List<Stream> streams = new();
+        List<Stream> streams = [];
         try
         {
             foreach (var base64Pdf in request.Base64Pdfs)
@@ -97,18 +99,166 @@ public class PdfOperationsController(IMediator mediator) : ControllerBase
 
         return File(mergedPdf, "application/pdf", "merged.pdf");
     }
+
+    /// <summary>
+    /// Adds an annotation to a PDF document.
+    /// Supports multipart/form-data file upload.
+    /// </summary>
+    /// <param name="request">Multipart form data containing PDF file and annotation parameters</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Annotated PDF file</returns>
+    /// <response code="200">Annotation added successfully - returns annotated PDF</response>
+    /// <response code="400">Invalid input (invalid page number, missing required parameters)</response>
+    /// <response code="500">Internal server error during PDF processing</response>
+    [HttpPost("annotate", Name = "AnnotateFromFile")]
+    [Consumes("multipart/form-data")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> AnnotateFromFile(
+        [FromForm] AddAnnotationMultipartRequest request,
+        CancellationToken cancellationToken)
+    {
+        var command = new AddAnnotationCommand
+        {
+            PdfStream = request.File.OpenReadStream(),
+            AnnotationType = request.AnnotationType,
+            PageNumber = request.PageNumber,
+            Rectangle = (request.X1, request.Y1, request.X2, request.Y2),
+            Content = request.Content,
+            Author = request.Author,
+            Color = request.Color,
+            TextMarkupStyle = request.TextMarkupStyle
+        };
+
+        byte[] annotatedPdf = await mediator.Send(command, cancellationToken);
+
+        return File(annotatedPdf, "application/pdf", "annotated.pdf");
+    }
+
+    /// <summary>
+    /// Adds an annotation to a PDF document.
+    /// Supports Base64-encoded PDF via JSON payload.
+    /// </summary>
+    /// <param name="command">Command containing all annotation parameters (including Base64 PDF)</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Annotated PDF file</returns>
+    /// <response code="200">Annotation added successfully - returns annotated PDF</response>
+    /// <response code="400">Invalid input (Base64 format error, invalid page number, missing required parameters)</response>
+    /// <response code="500">Internal server error during PDF processing</response>
+    [HttpPost("annotate", Name = "AnnotateFromBase64")]
+    [Consumes("application/json")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> AnnotateFromBase64(
+        [FromBody] AddAnnotationBase64Command command,
+        CancellationToken cancellationToken)
+    {
+        // Convert Base64 to MemoryStream
+        Stream pdfStream;
+        try
+        {
+            byte[] pdfBytes = Convert.FromBase64String(command.Base64Pdf);
+            pdfStream = new MemoryStream(pdfBytes);
+        }
+        catch (FormatException ex)
+        {
+            throw new BadRequestException("Invalid Base64 format: " + ex.Message);
+        }
+
+        var annotationCommand = new AddAnnotationCommand
+        {
+            PdfStream = pdfStream,
+            AnnotationType = command.AnnotationType,
+            PageNumber = command.PageNumber,
+            Rectangle = (command.X1, command.Y1, command.X2, command.Y2),
+            Content = command.Content,
+            Author = command.Author,
+            Color = command.Color,
+            TextMarkupStyle = command.TextMarkupStyle
+        };
+
+        byte[] annotatedPdf = await mediator.Send(annotationCommand, cancellationToken);
+
+        // Cleanup stream
+        pdfStream.Dispose();
+
+        return File(annotatedPdf, "application/pdf", "annotated.pdf");
+    }
 }
 
 /// <summary>
-/// Request DTO for Base64-encoded PDF merge operation
+/// Request DTO for multipart/form-data annotation operation
 /// </summary>
-public record MergePdfsRequest
+public class AddAnnotationMultipartRequest
+{
+    /// <summary>
+    /// PDF file to annotate
+    /// </summary>
+    public required IFormFile File { get; set; }
+
+    /// <summary>
+    /// Type of annotation (TextMarkup, FreeText, StickyNote, Circle, Square)
+    /// </summary>
+    public required AnnotationType AnnotationType { get; set; }
+
+    /// <summary>
+    /// Target page number (1-indexed)
+    /// </summary>
+    public required int PageNumber { get; set; }
+
+    /// <summary>
+    /// Rectangle X1 coordinate
+    /// </summary>
+    public required double X1 { get; set; }
+
+    /// <summary>
+    /// Rectangle Y1 coordinate
+    /// </summary>
+    public required double Y1 { get; set; }
+
+    /// <summary>
+    /// Rectangle X2 coordinate
+    /// </summary>
+    public required double X2 { get; set; }
+
+    /// <summary>
+    /// Rectangle Y2 coordinate
+    /// </summary>
+    public required double Y2 { get; set; }
+
+    /// <summary>
+    /// Annotation content (required for FreeText/StickyNote)
+    /// </summary>
+    public string? Content { get; set; }
+
+    /// <summary>
+    /// Author name (optional)
+    /// </summary>
+    public string? Author { get; set; }
+
+    /// <summary>
+    /// Hex color (6 digits, e.g., "FF0000" for red)
+    /// </summary>
+    public string? Color { get; set; }
+
+    /// <summary>
+    /// Markup style (Highlight/Underline/Strikeout, required for TextMarkup)
+    /// </summary>
+    public TextMarkupStyle? TextMarkupStyle { get; set; }
+}
+
+/// <summary>
+/// Request DTO for Base64-encoded PDF merge operation (API layer only - converts to MergePdfsCommand)
+/// </summary>
+public record MergePdfsBase64Request
 {
     /// <summary>
     /// Array of Base64-encoded PDF files (minimum 2 required)
     /// </summary>
     /// <example>["JVBERi0xLjQK...", "JVBERi0xLjQK..."]</example>
-    public List<string> Base64Pdfs { get; init; } = new();
+    public required List<string> Base64Pdfs { get; init; }
 
     /// <summary>
     /// Optional page ranges per PDF (null = all pages). 
@@ -118,3 +268,75 @@ public record MergePdfsRequest
     /// <example>["1-2", "1,3,5", null]</example>
     public List<string?>? PageRanges { get; init; }
 }
+
+/// <summary>
+/// Request DTO for Base64-encoded PDF annotation (API layer only - converts to AddAnnotationCommand)
+/// </summary>
+public record AddAnnotationBase64Command
+{
+    /// <summary>
+    /// Base64-encoded PDF file
+    /// </summary>
+    /// <example>"JVBERi0xLjQK..."</example>
+    public required string Base64Pdf { get; init; }
+
+    /// <summary>
+    /// Type of annotation to add
+    /// </summary>
+    /// <example>TextMarkup</example>
+    public required AnnotationType AnnotationType { get; init; }
+
+    /// <summary>
+    /// Target page number (1-indexed)
+    /// </summary>
+    /// <example>1</example>
+    public required int PageNumber { get; init; }
+
+    /// <summary>
+    /// Rectangle X1 coordinate
+    /// </summary>
+    /// <example>100.0</example>
+    public required double X1 { get; init; }
+
+    /// <summary>
+    /// Rectangle Y1 coordinate
+    /// </summary>
+    /// <example>100.0</example>
+    public required double Y1 { get; init; }
+
+    /// <summary>
+    /// Rectangle X2 coordinate
+    /// </summary>
+    /// <example>200.0</example>
+    public required double X2 { get; init; }
+
+    /// <summary>
+    /// Rectangle Y2 coordinate
+    /// </summary>
+    /// <example>120.0</example>
+    public required double Y2 { get; init; }
+
+    /// <summary>
+    /// Annotation content (required for FreeText and StickyNote)
+    /// </summary>
+    /// <example>"Important text to highlight"</example>
+    public string? Content { get; init; }
+
+    /// <summary>
+    /// Author name (optional)
+    /// </summary>
+    /// <example>"John Doe"</example>
+    public string? Author { get; init; }
+
+    /// <summary>
+    /// Hex color (6 digits, e.g., "FF0000" for red). Optional - defaults vary by annotation type.
+    /// </summary>
+    /// <example>"FFFF00"</example>
+    public string? Color { get; init; }
+
+    /// <summary>
+    /// Text markup style (Highlight, Underline, or Strikeout). Required for TextMarkup annotations.
+    /// </summary>
+    /// <example>Highlight</example>
+    public TextMarkupStyle? TextMarkupStyle { get; init; }
+}