title: Add field for a digital signature to PDF
body: <p>I create a new PDF file using PDFsharp and MigraDoc.
  Now I want to add a field where the user can click on it and select a certificate for digital signing the file.</p>
  
  <p>I found in the web, that this should be possible with AcroForms.
  But I wasn't able to use <code>AcroForm</code> because it is always null.</p>
  
  <p>My current example code:</p>
  
  <pre><code>Document document = new Document();
  
  Section section = document.AddSection();
  section.AddParagraph("Signature Test");
  
  
  PdfDocumentRenderer pdfRenderer = new PdfDocumentRenderer(false, PdfFontEmbedding.Always);
  pdfRenderer.Document = document;
  pdfRenderer.RenderDocument();
  
  // NullPointerException at the following line. AcroForm is null 
  pdfRenderer.PdfDocument.AcroForm.Elements.Add(PdfAcroForm.Keys.SigFlags, new PdfInteger(3));
  
  const string filename = "HelloWorld.pdf";
  pdfRenderer.PdfDocument.Save(filename);
  Process.Start(filename);
  </code></pre>
  
  <p>Why is this property null? What can I do to set this to a correct value?</p>
  
  <p>Or better, how can I add a field to select the digital certificate?</p>