diff --git a/DocumentOperator.Application/Common/Interfaces/IPdfProcessor.cs b/DocumentOperator.Application/Common/Interfaces/IPdfProcessor.cs
index dc914ce..667c07b 100644
--- a/DocumentOperator.Application/Common/Interfaces/IPdfProcessor.cs
+++ b/DocumentOperator.Application/Common/Interfaces/IPdfProcessor.cs
@@ -58,4 +58,22 @@ public interface IPdfProcessor
     /// Thrown when PDF contains no attachments
     /// </exception>
     Task<byte[]> ExtractAttachmentsAsync(Stream pdfStream);
+
+    /// <summary>
+    /// Merges multiple PDF documents into a single PDF.
+    /// </summary>
+    /// <param name="pdfStreams">
+    /// PDF streams to merge (minimum 2 required). Each stream must be readable and positioned 
+    /// at the beginning (Position = 0). Caller is responsible for disposal.
+    /// </param>
+    /// <param name="pageRanges">
+    /// Optional page ranges per PDF (null = all pages). Format: "1-3,5" means pages 1, 2, 3, and 5.
+    /// If null or empty for a PDF, all pages are included. Array length must match pdfStreams length if provided.
+    /// </param>
+    /// <returns>Merged PDF as byte array</returns>
+    /// <exception cref="Domain.Common.Exceptions.BadRequestException">
+    /// Thrown when fewer than 2 PDFs provided, any stream is empty/invalid/not at Position=0, 
+    /// or page range format is invalid
+    /// </exception>
+    Task<byte[]> MergePdfsAsync(IReadOnlyList<Stream> pdfStreams, IReadOnlyList<string?>? pageRanges = null);
 }
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