Code:
string appPath = Path.GetDirectoryName(System.Reflection.Assembly.GetEntryAssembly().Location);
            string fileNameExisting = Path.Combine(appPath, "template.dat");
            string fileNameNew = destName;

            using (var existingFileStream = new FileStream(fileNameExisting, FileMode.Open))
            using (var newFileStream = new FileStream(fileNameNew, FileMode.Create))
            {
                // Open existing PDF  
                var pdfReader = new PdfReader(existingFileStream);

                // PdfStamper, which will create  
                var stamper = new PdfStamper(pdfReader, newFileStream);

                var form = stamper.AcroFields;
                var fieldKeys = form.Fields.Keys;

                form.SetField("name", patientName);
                form.SetField("facility", facility);
                form.SetField("refer", provider);
                form.SetField("exam", examVal);
                form.SetField("patientid", xRayNo);
                form.SetField("dob", dob);
                form.SetField("dateofservice", dateOfService);
                form.SetField("data", data);
                form.SetField("signed", signedOnDate);
                form.SetField("dr", drName);


                // "Flatten" the form so it wont be editable/usable anymore  
                stamper.FormFlattening = true;

                stamper.Close();
                pdfReader.Close();
            }
        }