test.cs

string xmlZugferd = ""; // your XML
string metaData = ""; // required RDF meta data

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

// create a new embedded file
var zugferdInvoice = new TXTextControl.EmbeddedFile(
  "ZUGFeRD-invoice.xml",
  Encoding.UTF8.GetBytes(xmlZugferd),
  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[] {
  new TXTextControl.EmbeddedFile(
     "ZUGFeRD-invoice.xml",
     Encoding.UTF8.GetBytes(xmlZugferd),
     metadata) };

// export the PDF
textControl1.Save("test.pdf", TXTextControl.StreamType.AdobePDFA, saveSettings);