test.cs

public string UpdateDocument(string DocumentName, byte[] document) {

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

    // the load PDF document
    TXTextControl.LoadSettings loadSettings = new LoadSettings();
    tx.Load(DocumentName,
      TXTextControl.StreamType.AdobePDF,
      loadSettings);

    List<EmbeddedFile> embeddedFiles = loadSettings.EmbeddedFiles.ToList();

    // create an attachment
    EmbeddedFile embeddedFile = new EmbeddedFile("original.tx", document, null);
    embeddedFile.Relationship = "Source";

    embeddedFiles.Add(embeddedFile);

    TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings() {
      EmbeddedFiles = embeddedFiles.ToArray()
    };

    tx.Load(document, BinaryStreamType.InternalUnicodeFormat);

    // save a PDF with the attached Text Control document embedded
    tx.Save(DocumentName,
      TXTextControl.StreamType.AdobePDF,
      saveSettings);
  }

  return DocumentName;
}