test.cs

public static class Invoice {
  public static byte[] Create(Order order) {

    TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings();

    var metaData = System.IO.File.ReadAllText("metadata.xml");

    // create a new embedded file
    var zugferdInvoice = new TXTextControl.EmbeddedFile(
      "ZUGFeRD-invoice.xml",
      order.ZugferdXML,
      metaData);

    zugferdInvoice.Description = "ZUGFeRD-invoice";
    zugferdInvoice.Relationship = "Alternative";
    zugferdInvoice.MIMEType = "application/xml";
    zugferdInvoice.LastModificationDate = DateTime.Now;

    // set the embedded files
    saveSettings.EmbeddedFiles = new TXTextControl.EmbeddedFile[] { zugferdInvoice };

    byte[] document;

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

      tx.Load("template.tx", StreamType.InternalUnicodeFormat);

      using (MailMerge mailMerge = new MailMerge()) {
        mailMerge.TextComponent = tx;
        mailMerge.MergeObject(order);
      }

      // export the PDF
      tx.Save(out document, TXTextControl.BinaryStreamType.AdobePDFA, saveSettings);

      return document;
    }
  }
}