tx.cs

// create a non-UI ServerTextControl instance
using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl()) {

  tx.Create();
  // set dummy content
  tx.Text = "PDF Document Content";

  // read the content of the attachment
  string sAttachment = System.IO.File.ReadAllText("attachment.txt");

  // create the attachement
  TXTextControl.EmbeddedFile attachment =
     new TXTextControl.EmbeddedFile(
        "attachment.txt",
        sAttachment,
        null) {
       Description = "My Text File",
       Relationship = "Unspecified",
       MIMEType = "application/txt",
       CreationDate = DateTime.Now,
     };

  // attached the embedded file
  tx.DocumentSettings.EmbeddedFiles =
     new TXTextControl.EmbeddedFile[] { attachment };

  // save as PDF/A
  tx.Save("document.pdf", TXTextControl.StreamType.AdobePDFA);
}