public void SavePdf()
{
    System.Text.Encoding.RegisterProvider(System.Text.CodePagesEncodingProvider.Instance);

    // Create a new PDF document
    PdfDocument pdfDocument = new PdfDocument();
    pdfDocument.Version = 15;

    // Create an empty page
    PdfPage pdfPage;
    pdfPage = pdfDocument.AddPage();
    pdfPage.Width = XUnit.FromInch(8.5);
    pdfPage.Height = XUnit.FromInch(11.00);

    // Get an XGraphics object for drawing
    XGraphics gfx = XGraphics.FromPdfPage(pdfPage);

    // Creating the Rubber Stamp with some default settings
    PdfRubberStampAnnotation cstamp = new PdfRubberStampAnnotation() { Icon = PdfRubberStampAnnotationIcon.TopSecret };
    XRect rect = gfx.Transformer.WorldToDefaultPage(new XRect(new XPoint(10, 10), new XSize(200, 50)));
    cstamp.Rectangle = new PdfRectangle(rect);
    cstamp.Color = XColor.FromArgb(255, 0, 0);

    // Load the custom image
    var image = XImage.FromFile(@"c:\temp\test.bmp");
    var pdfImage = new PdfImage(pdfDocument, image);
    string r = pdfPage.Resources.AddImage(pdfImage); // adding the image to the Page References

    // Now let's create the XForm object, and reference the image
    var matrix = new PdfArray();
    matrix.Elements.Add(new PdfReal(1));
    matrix.Elements.Add(new PdfReal(0));
    matrix.Elements.Add(new PdfReal(0));
    matrix.Elements.Add(new PdfReal(1));
    matrix.Elements.Add(new PdfReal(0));
    matrix.Elements.Add(new PdfReal(0));

    var xform = new PdfDictionary(pdfDocument);
    xform.Elements.Add("/Type", new PdfName("/XObject"));
    xform.Elements.Add("/Subtype", new PdfName("/Form"));
    xform.Elements.Add("/FormType", new PdfInteger(1));
    xform.Elements.Add("/BBox", new PdfRectangle(new XRect(10, 10, image.PointWidth, image.PointHeight)));
    xform.Elements.Add("/Matrix", matrix);
    xform.Elements.Add("/Resources", pdfImage.Reference);
    xform.Elements.Add("/Length", new PdfInteger(0));

    // Now let's create the appearance stream, and add the XForm object
    var appearanceStream = new PdfDictionary(pdfDocument);
    appearanceStream.Elements.Add("/N", xform);
    cstamp.Elements.Add("/AP", appearanceStream);

    // Add to the page
    gfx.PdfPage.Annotations.Add(cstamp);

    pdfDocument.Save(@"c:\temp\1.pdf");
}