HomeController.cs

[HttpPost]
public string SignDocument(Document model)
{
    byte[] document = 
        Convert.FromBase64String(model.BinaryDocument);
    byte[] signatureDocument = null;

    using (ServerTextControl tx = new ServerTextControl())
    {
        tx.Create();

        // use MailMerge to merge the signature template
        using (MailMerge mailMerge = new MailMerge())
        {
            mailMerge.TextComponent = tx;
            mailMerge.SearchPath = Server.MapPath("/Signature_Images/");
            mailMerge.LoadTemplate(
                Server.MapPath("/App_Data/documents/signature_template.tx"), 
                FileFormat.InternalUnicodeFormat);

            // create a new signature object
            Signature signature = new Signature();
            signature.Name = "Peter Chadwick";
            signature.SignatureImage = 
                signature.Name.ToLower().Replace(" ", "_") + ".png";

            // merge and save the resulting document
            mailMerge.MergeObject(signature);
            mailMerge.SaveDocumentToMemory(
                out signatureDocument, 
                BinaryStreamType.InternalUnicodeFormat, 
                null);
        }

        // load the original document from the editor
        tx.Load(document, BinaryStreamType.InternalUnicodeFormat);

        // find the "signature" text frame with the name "txsig"
        foreach (TextFrame frame in tx.TextFrames)
        {
            if (frame.Name == "txsig")
            {
                frame.Tables.Clear();
                frame.Selection.Start = 0;
                frame.Selection.Length = -1;
                
                // load the merged signature template into the
                // text frame and save the complete document
                frame.Selection.Load(signatureDocument, 
                    BinaryStreamType.InternalUnicodeFormat);
                tx.Save(out document, BinaryStreamType.InternalUnicodeFormat);
                break;
            }
        }
        
    }

    // finally, return the signed document
    return Convert.ToBase64String(document);
}