createpdf.cs

var sData = File.ReadAllText("data.xml");

TXTextControl.EmbeddedFile file = new TXTextControl.EmbeddedFile("data.xml", sData, "") {
    MIMEType = "text/xml",
    RelationShip = "Alternative"
};

TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings() {
    EmbeddedFiles = new TXTextControl.EmbeddedFile[] { file }
};

textControl1.Save("myinvoice.pdf", 
    TXTextControl.StreamType.AdobePDFA, 
    saveSettings);