diff --git a/DocumentOperator.Tests/Unit/Infrastructure/Services/PdfProcessing/DevExpressPdfProcessorTests.cs b/DocumentOperator.Tests/Unit/Infrastructure/Services/PdfProcessing/DevExpressPdfProcessorTests.cs
index 7aed60b..c6616b8 100644
--- a/DocumentOperator.Tests/Unit/Infrastructure/Services/PdfProcessing/DevExpressPdfProcessorTests.cs
+++ b/DocumentOperator.Tests/Unit/Infrastructure/Services/PdfProcessing/DevExpressPdfProcessorTests.cs
@@ -565,5 +565,252 @@ public class DevExpressPdfProcessorTests
     }
 
     #endregion
+
+    #region AddAnnotationAsync Tests
+
+    [Fact]
+    public async Task AddAnnotationAsync_TextMarkupHighlight_ReturnsAnnotatedPdf()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+        var annotationType = Domain.Models.ValueObjects.AnnotationType.TextMarkup;
+        var rectangle = (X1: 100.0, Y1: 100.0, X2: 200.0, Y2: 120.0);
+        var textMarkupStyle = Domain.Models.ValueObjects.TextMarkupStyle.Highlight;
+
+        // Act
+        byte[] result = await _sut.AddAnnotationAsync(
+            pdfStream,
+            annotationType,
+            pageNumber: 1,
+            rectangle,
+            content: "Important text",
+            author: "Test Author",
+            color: "FFFF00", // Yellow
+            textMarkupStyle);
+
+        // Assert
+        result.Should().NotBeEmpty();
+        result.Length.Should().BeGreaterThan((int)pdfStream.Length); // Annotation adds bytes
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_FreeText_ReturnsAnnotatedPdf()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+        var annotationType = Domain.Models.ValueObjects.AnnotationType.FreeText;
+        var rectangle = (X1: 50.0, Y1: 50.0, X2: 150.0, Y2: 100.0);
+
+        // Act
+        byte[] result = await _sut.AddAnnotationAsync(
+            pdfStream,
+            annotationType,
+            pageNumber: 1,
+            rectangle,
+            content: "Free text annotation",
+            author: "John Doe",
+            color: "FF0000"); // Red
+
+        // Assert
+        result.Should().NotBeEmpty();
+        result.Length.Should().BeGreaterThan((int)pdfStream.Length);
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_StickyNote_ReturnsAnnotatedPdf()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+        var annotationType = Domain.Models.ValueObjects.AnnotationType.StickyNote;
+        var rectangle = (X1: 300.0, Y1: 300.0, X2: 320.0, Y2: 320.0);
+
+        // Act
+        byte[] result = await _sut.AddAnnotationAsync(
+            pdfStream,
+            annotationType,
+            pageNumber: 1,
+            rectangle,
+            content: "Please review this section",
+            author: "Reviewer");
+
+        // Assert
+        result.Should().NotBeEmpty();
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_Circle_ReturnsAnnotatedPdf()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+        var annotationType = Domain.Models.ValueObjects.AnnotationType.Circle;
+        var rectangle = (X1: 100.0, Y1: 200.0, X2: 200.0, Y2: 300.0);
+
+        // Act
+        byte[] result = await _sut.AddAnnotationAsync(
+            pdfStream,
+            annotationType,
+            pageNumber: 1,
+            rectangle,
+            content: "Circle annotation",
+            color: "00FF00"); // Green
+
+        // Assert
+        result.Should().NotBeEmpty();
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_Square_ReturnsAnnotatedPdf()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+        var annotationType = Domain.Models.ValueObjects.AnnotationType.Square;
+        var rectangle = (X1: 250.0, Y1: 250.0, X2: 350.0, Y2: 350.0);
+
+        // Act
+        byte[] result = await _sut.AddAnnotationAsync(
+            pdfStream,
+            annotationType,
+            pageNumber: 1,
+            rectangle,
+            color: "0000FF"); // Blue
+
+        // Assert
+        result.Should().NotBeEmpty();
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_EmptyStream_ThrowsBadRequestException()
+    {
+        // Arrange
+        using var emptyStream = new MemoryStream();
+
+        // Act
+        var act = async () => await _sut.AddAnnotationAsync(
+            emptyStream,
+            Domain.Models.ValueObjects.AnnotationType.FreeText,
+            pageNumber: 1,
+            rectangle: (0, 0, 100, 100),
+            content: "Test");
+
+        // Assert
+        await act.Should().ThrowAsync<BadRequestException>()
+            .WithMessage("*cannot be empty*");
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_StreamNotAtPositionZero_ThrowsBadRequestException()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+        pdfStream.Position = 10; // Move stream position
+
+        // Act
+        var act = async () => await _sut.AddAnnotationAsync(
+            pdfStream,
+            Domain.Models.ValueObjects.AnnotationType.Circle,
+            pageNumber: 1,
+            rectangle: (0, 0, 100, 100));
+
+        // Assert
+        await act.Should().ThrowAsync<BadRequestException>()
+            .WithMessage("*must be at position 0*");
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_InvalidPageNumber_ThrowsBadRequestException()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+
+        // Act
+        var act = async () => await _sut.AddAnnotationAsync(
+            pdfStream,
+            Domain.Models.ValueObjects.AnnotationType.FreeText,
+            pageNumber: 999, // Exceeds page count
+            rectangle: (0, 0, 100, 100),
+            content: "Test");
+
+        // Assert
+        await act.Should().ThrowAsync<BadRequestException>()
+            .WithMessage("*exceeds document page count*");
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_FreeTextWithoutContent_ThrowsBadRequestException()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+
+        // Act
+        var act = async () => await _sut.AddAnnotationAsync(
+            pdfStream,
+            Domain.Models.ValueObjects.AnnotationType.FreeText,
+            pageNumber: 1,
+            rectangle: (0, 0, 100, 100),
+            content: null); // Missing required content
+
+        // Assert
+        await act.Should().ThrowAsync<BadRequestException>()
+            .WithMessage("*requires content*");
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_StickyNoteWithoutContent_ThrowsBadRequestException()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+
+        // Act
+        var act = async () => await _sut.AddAnnotationAsync(
+            pdfStream,
+            Domain.Models.ValueObjects.AnnotationType.StickyNote,
+            pageNumber: 1,
+            rectangle: (0, 0, 20, 20),
+            content: ""); // Empty content
+
+        // Assert
+        await act.Should().ThrowAsync<BadRequestException>()
+            .WithMessage("*requires content*");
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_TextMarkupWithoutStyle_ThrowsBadRequestException()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+
+        // Act
+        var act = async () => await _sut.AddAnnotationAsync(
+            pdfStream,
+            Domain.Models.ValueObjects.AnnotationType.TextMarkup,
+            pageNumber: 1,
+            rectangle: (0, 0, 100, 100),
+            textMarkupStyle: null); // Missing required style
+
+        // Assert
+        await act.Should().ThrowAsync<BadRequestException>()
+            .WithMessage("*requires textMarkupStyle*");
+    }
+
+    [Fact]
+    public async Task AddAnnotationAsync_InvalidColorFormat_ThrowsBadRequestException()
+    {
+        // Arrange
+        using var pdfStream = LoadTestPdfAsStream("valid.pdf");
+
+        // Act
+        var act = async () => await _sut.AddAnnotationAsync(
+            pdfStream,
+            Domain.Models.ValueObjects.AnnotationType.Circle,
+            pageNumber: 1,
+            rectangle: (0, 0, 100, 100),
+            color: "INVALID"); // Invalid hex format
+
+        // Assert
+        await act.Should().ThrowAsync<BadRequestException>()
+            .WithMessage("*Invalid color format*");
+    }
+
+    #endregion
 }