Я нашел способ сделать это через поля формы PDF.
String formFile = Server.MapPath("~/") + "source.pdf";
String newFile = Server.MapPath("~/") + "sink.pdf";
PdfReader reader = new PdfReader(formFile);
PdfStamper stamper = new PdfStamper(reader, new FileStream(newFile, FileMode.Create));
AcroFields fields = stamper.AcroFields;
// set form fields
fields.SetField("{TO}", "John Doe");
fields.SetField("{FROM}", "2 Milky Way, London");
stamper.FormFlattening = true;
stamper.Close();