Once, I've got similar issue on certain hosting due to some apache extension that forbids to have files with write permission for anyone else than owner. Please investigate server logs. PHP logs as Janusz suggested may not show cause, because probably it's level deeper, than PHP.
PHP is running on HTTP server, and most of 500 errors are caused by some settings or requirements of server that are not satisfied by script (EPESI).
If you have panel for administrating your hosting look for "server logs" or similar. If it's your own server then it depends on configuration. It may be in /var/log/http, /var/log/apache, /var/www/site/logs/, etc
Regards,
Adam
PS. Maybe just you reach your memory_limit? PDF generating uses a lot of memory.