test.cs

[HttpGet]
public string PDFPrepare(bool flatten) {
   byte[] bDocument;

   // create temporary ServerTextControl
   using (ServerTextControl tx = new ServerTextControl()) {
      tx.Create();

      // loading the template
      tx.Load(Server.MapPath("~/App_Data/Documents/patient_history.tx"), StreamType.InternalUnicodeFormat);

      // fire up the MailMerge engine
      using (MailMerge mailMerge = new MailMerge()) {
         mailMerge.TextComponent = tx; // connect to ServerTextControl

         // pre-selecting form fields         
         if (flatten == false)
            mailMerge.FormFieldMergeType = FormFieldMergeType.Preselect;
         else // flatten form fields
            mailMerge.FormFieldMergeType = FormFieldMergeType.Replace;

         string jsonData = System.IO.File.ReadAllText(Server.MapPath("~/App_Data/Documents/data.json"));
         
         // merge JSON into fields
         mailMerge.MergeJsonData(jsonData);
      }

      // export to PDF
      tx.Save(out bDocument, BinaryStreamType.InternalUnicodeFormat);
   }

   // return as Base64 encoded string
   return Convert.ToBase64String(bDocument);
}