Comment 46 for bug 147551

In my system, a Gutsy, the problem was solved by editing

  /etc/apparmor.d/usr.sbin.cupsd

and changing the entry that read:

/usr/lib/cups/backend/cups-pdf {

to:

/var/lib/cups/backend/cups-pdf {

Since that was the path to the executable which apparmor complained about violations in /var/log/kern.log

Reading /var/log/kern.log is the ticket out of this issue.