After I upgraded my Ubuntu box to the latest version a PDF printer no longer worked.
Trying to print to pdf I noticed the following message:
“Backend /usr/lib/cups/backend/cups-pdf does not exist!”
I also noticed, that the printer was not enabled in Printer Properties under Polices.

To fix it I had to do the following:
1. Reinstall cups-pdf

sudo apt-get purge cups-pdf
sudo apt-get install cups-pdf

note: you have to purge cups-pdf, simple installation does not work.

2. Enable PDF printer

Right click on PDF printer -> Properties -> Policies -> check “Enabled”, Click “Apply”.