Comment 6 for bug 487629

After careful examination, it appears that 3 superfluous lines in the PPD were added by the "PDF Workflow" patch that was used for 2.5.0 releases, but has been removed since CUPS-PDF 2.5.1. However, the PPD was not automatically upgraded by our package's maintainer script, which is probably what broke things for people upgrading to a 2.5.1 release. This has been fixed as of CUPS-PDF 2.5.1-4 which entered Oneiric today, by utilizing a feature of CUPS 1.5.0-3 and more recent to auto-update PPD files whenever a new version gets installed.

As a crude test, for older releases, could you try purging and re-installing the package and see if it fixes it?

sudo dpkg -P cups-pdf && sudo apt-get install cups-pdf