Comment 69 for bug 295536

Ok, I believe I have solved this problem (at least in Jaunty as I haven't tried in other versions).

1. run "sudo nano /etc/apparmor.d/usr.sbin.cupsd"
2. Add "capability dac_read_search," (no quotes and don't forget the comma at the end)
NOTE: It MUST be in the /usr/lib/cups/backend/cups-pdf section at the end. (I added it below "capability setuid,")
3. Save file and delete any backup files created if you used a different editor (or move to some other directory).
4. run "sudo /etc/init.d/apparmor stop" (no quotes)
5. run "/etc/init.d/apparmor start" (no quotes)

This worked for me.
After I found that aa-complain worked I looked at dmesg and found that apparmor was complaining about dac_read_search. I tried adding that capability in other places but it only worked as decribed above. I was sure to reset using aa-enforce to verify. I'm pretty sure this is the problem. Of course you will still have to create ~/PDF.

Hopefully this will resove your problem!