Look into /etc/cups/cups-pdf. The individual spool directories must have
proper access rights (like .ssh in your home) and match the UserUMask
setting. Otherwise cups-pdf will not generate the PDF, simply because others
might access the directory also.

With all settings as default, it should look like this:

$ ll /var/spool/cups-pdf/
drwxr-xr-x 6 root   root   54 2007-10-01 19:01 ./
drwxr-xr-x 7 root   root   78 2007-08-25 16:50 ../
drwx------ 2 jdoe   lp      6 2007-10-01 19:01 jdoe/
drwxr-x--x 2 root   lp      6 2010-03-01 13:41 SPOOL/


- Jörg