This is the only way to ensure that we always have the latest PPD installed for older releases. Happily, starting with Oneiric, we make use of a new feature in CUPS to automatically update the PPD at install time. Unless this new CUPS feature can be backported, I see no easy solution other than manually purging and re-installing as above.
I'm affraid that for releases older that Oneiric, there isn't any answer other than purging and re-installing the package:
sudo dpkg -P cups-pdf && sudo apt-get install cups-pdf
This is the only way to ensure that we always have the latest PPD installed for older releases. Happily, starting with Oneiric, we make use of a new feature in CUPS to automatically update the PPD at install time. Unless this new CUPS feature can be backported, I see no easy solution other than manually purging and re-installing as above.