We've been investigating the latest version of GroupDocs.Signature for .net using your example console applications in GitHub (https://github.com/groupdocs-signature/GroupDocs.Signature-for.NET).
When we "digitally" sign a PDF document, then open the resulting file in Adobe Reader XI, the certification block field is detected and seems to work correctly. However, in our product workflow, multiple people may be required to sign the same document. PDFs do not appear to accept having a document certified multiple times (the previous certification that becomes invalid causes a big "not valid" error at the top of the page).
Is there any way to place Signature field(s) before certifying the document, so that multiple people can sign? ("Sign with text" appears to just place a text field, rather than being a signature field)