I believe after referring this thread you will get the results.
The below thread was issue with Printer but I think the same can be used for converting to PDF also.
Refer following notes:
559323 PDF conversion: Thai support for ABAP lists
339832 PDF conversion: Thai support
794610 PDF output of Thai text in Unicode
807688 PDF-Conversion of Thai ABAP-Lists: wrong alignment
425279 Thai support for PDF conversion on AS/400
Also refer :