Print the visual representation of a digital signature

src/foundation/src/PDFsharp/src/PdfSharp/Pdf.Signatures/DigitalSignatureHandler.cs

@@ -218,6 +218,10 @@ PdfFormSignatureField GetSignatureField(PdfSignature signatureDic) // #US321 TOD
 
             signatureField.Elements.Add(PdfAnnotation.Keys.Rect, new PdfRectangle(Options.Rectangle));
 
+            // Without the print flag, a viewer never prints the annotation, so the visual representation
+            // of the signature would be missing on paper. See 12.5.3 Annotation flags.
+            signatureField.Elements.Add(PdfAnnotation.Keys.F, new PdfInteger((int)PdfAnnotationFlags.Print));
+
             // TODO COMPILE
             signatureField.CustomAppearanceHandler = Options.AppearanceHandler ?? new DefaultSignatureAppearanceHandler()
             {

src/foundation/src/PDFsharp/tests/PdfSharp.Tests/Pdf.Signatures/SignatureAppearanceTests.cs

@@ -0,0 +1,101 @@
+// PDFsharp - A .NET library for processing PDF
+// See the LICENSE file in the solution root for more information.
+
+#if WPF
+using System.IO;
+#endif
+using FluentAssertions;
+using PdfSharp.Diagnostics;
+using PdfSharp.Drawing;
+using PdfSharp.Pdf;
+using PdfSharp.Pdf.Annotations;
+using PdfSharp.Pdf.Forms;
+using PdfSharp.Pdf.IO;
+using PdfSharp.Pdf.Signatures;
+#if CORE
+using PdfSharp.Fonts;
+using PdfSharp.Quality;
+#endif
+using Xunit;
+
+namespace PdfSharp.Tests.Pdf.Signatures
+{
+    [Collection("PDFsharp")]
+    public class SignatureAppearanceTests : IDisposable
+    {
+        public SignatureAppearanceTests()
+        {
+            PdfSharpCore.ResetAll();
+#if CORE
+            GlobalFontSettings.FontResolver = new UnitTestFontResolver();
+#endif
+        }
+
+        public void Dispose()
+        {
+            PdfSharpCore.ResetAll();
+        }
+
+        [Fact]
+        public void Signature_is_printed()
+        {
+            using var document = new PdfDocument();
+            document.AddPage();
+
+            var options = new DigitalSignatureOptions
+            {
+                Reason = "License Agreement",
+                Rectangle = new XRect(36, 36, 200, 50),
+                AppearanceHandler = new EmptyAppearanceHandler()
+            };
+            _ = DigitalSignatureHandler.ForDocument(document, new TestSigner(), options);
+
+            using var stream = new MemoryStream();
+            document.Save(stream, false);
+
+            using var signedDocument = PdfReader.Open(new MemoryStream(stream.ToArray()), PdfDocumentOpenMode.Import);
+            var field = signedDocument.Catalog.GetAcroForm()!.Elements
+                .GetArray(PdfForm.Keys.Fields)!.Elements.GetRequiredDictionary(0);
+
+            var flags = (PdfAnnotationFlags)field.Elements.GetInteger(PdfAnnotation.Keys.F);
+            flags.Should().HaveFlag(PdfAnnotationFlags.Print,
+                "a signature that is not printed is missing on the paper the document is printed on");
+            flags.Should().NotHaveFlag(PdfAnnotationFlags.Hidden);
+            flags.Should().NotHaveFlag(PdfAnnotationFlags.NoView);
+        }
+
+        /// <summary>
+        /// A signer that creates a deterministic dummy signature, so that no certificate is needed.
+        /// </summary>
+        class TestSigner : IDigitalSigner
+        {
+            public string CertificateName => "PDFsharp unit test";
+
+            public Task<int> GetSignatureSizeAsync() => Task.FromResult(SignatureSize);
+
+            public Task<byte[]> GetSignatureAsync(Stream stream)
+            {
+                // Read the stream to ensure the ranges to be signed are readable.
+                var buffer = new byte[4096];
+                while (stream.Read(buffer, 0, buffer.Length) > 0)
+                { }
+
+                var signature = new byte[SignatureSize];
+                for (int idx = 0; idx < signature.Length; idx++)
+                    signature[idx] = (byte)idx;
+                return Task.FromResult(signature);
+            }
+
+            const int SignatureSize = 512;
+        }
+
+        /// <summary>
+        /// An appearance handler that draws nothing, so that no font is needed.
+        /// </summary>
+        class EmptyAppearanceHandler : IAnnotationAppearanceHandler
+        {
+            public void DrawAppearance(XGraphics gfx, XRect rect)
+            { }
+        }
+    }
+}