Comment 18 for bug 820820

Yes, CUPS turns everything to PDF (at least on Debian and Ubuntu), by using the filters texttopdf, imagetopdf, and pstopdf. In most cases these filters are not used at all as practically all desktop applications send print jobs in PDF. texttopdf usually only kicks in if a system admin wants to print quickly a config file with "lpr". All in all, one gets PDF and CUPS applies the pdftopdf filter to execute PPD-independent CUPS options like N-up, reverse order, .... After that the printer driver filters are called, pdftoraster and rasterto... for CUPS-Raster-based drivers, foomatic-rip for Ghostscript-based drivers, cpdftocps for PostScript printers. The driver filters apply the options from the PPD file, and some options of the PPD, like PageSize, are additionally used by the other filters.

For cups-pdf in my suggested architecture no driver filter is used. The PDF is then simply saved as it comes from CUPS or re-rendered by Ghostscript if desired by the user by the cups-pdf backend.