test.cs

public static void SaveDocument(SmartDocument smartDocument) {
  using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl()) {
    tx.Create();

    // load the edited document
    tx.Load(Convert.FromBase64String(smartDocument.Document), 
      TXTextControl.BinaryStreamType.InternalUnicodeFormat);

    // create an attachment from the edited document
    EmbeddedFile embeddedFile = new EmbeddedFile("original.tx",
      Convert.FromBase64String(smartDocument.Document),
      null);
    embeddedFile.Relationship = "Source";

    // attache the files
    TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings() {
      EmbeddedFiles = new EmbeddedFile[] { embeddedFile }
    };

    // save the document as PDF with the edited original attachment
    tx.Save("App_Data/" + smartDocument.Name, 
      TXTextControl.StreamType.AdobePDF,
      saveSettings);
  }
}