I am using itextsharp DLL for convert HTML to PDF. Some thailand words vowel not properly .please help me

using (var pdfWriter = PdfWriter.GetInstance(document, outputStream))
               {


                   pdfWriter.CloseStream = false;
                   document.SetPageSize(iTextSharp.text.PageSize.A3);

                   document.Open();
                   document.Add(objparagraph);
                   var worker = XMLWorkerHelper.GetInstance();
                   worker.ParseXHtml(pdfWriter, document, reader);
                   document.Close();
                   pdfWriter.Close();
               }

I am using itextsharp dll for convert html to PDF. some thailand words vowel not properly .please help me