test.cs

private string GetXmlAttachment(string Filename) {

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

    // load documment
    LoadSettings ls = new TXTextControl.LoadSettings() {
      PDFImportSettings = PDFImportSettings.LoadEmbeddedFiles
    };

    tx.Load(Filename, TXTextControl.StreamType.AdobePDF, ls);

    // all attachments
    var embeddedFiles = ls.EmbeddedFiles;

    // find the "alternative" xml representation
    foreach (EmbeddedFile embeddedFile in embeddedFiles) {

      if (embeddedFile.Relationship == "Alternative" &&
          embeddedFile.MIMEType     == "text/xml") {

        // return converted XML
        return Encoding.UTF8.GetString((byte[])embeddedFile.Data);
      }

    }

    return null; //something went wrong
  }
}