It's something related to conversion of whatever to pdf's - obviously. Assignments?
Cron job does many things. If you run the cron job from the command line via script in moodlecode/admin/cli/ ... php cron.php ... does it complete successfully or does it throw an error that provides a clue/hint? As not all crons will run at the same time, you might have to run that from CLI more than once.
Depending upon version of Moodle (that info not given), there are scheduled task that one might be able to refer for same 'hints/clues'.
'spirit of sharing', Ken