Hello apyash,
I recently had the same problem. In the php-Logs I could find out that osTicket tries to temporarily write the PDF-Files in C:\Windows\Temp. But since this directory is write protected for Users, it can't write the PDF-Files there. A possible workaround for this issue was to set write-access for all local users in the folder properties.
I couldn't find out how to manually set another path, which would be a more favourable solution, since the Temp directory is a system directory...
Hope this helps you anyway ;-)
CJ