Martin-Éric, due to the filter mechanisms of CUPS the data format
arriving at cups-pdf is always PostScript or PDF. The upstream PPD file
of cups-pdf forces CUPS to turn everything into PostScript, by omitting
any cupsFilter line which defaults to

*cupsFilter: "application/vnd.cups-postscript 0 -"

All conversion to PostScript is done by the CUPS filters, texttops for
plain text, pdftops for PDF, ... cups-pdf then receives PostScript and
nothing else.

In my suggested architecture the PPD file has

*cupsFilter: "application/vnd.cups-postscript 100 -"
*cupsFilter: "application/vnd.cups-pdf 0 -"

allowing both PostScript and PDF as input. Any other format is converted
by the filters coming with CUPS, so it is guaranteed that the input is
PostScript or PDF. PDF is the desired destination format, so if the
incoming format is PDF and the user has no special desires one could
simply save it. PostScript can be turned to PDF by the backend using
Ghostscript, as it was done before. If the user expresses special
desires via the config file, the input data is passed through
Ghostscript. In this case it does not matter whether the input data is
PostScript or PDF, as Ghostscript understands both and switches
automatically.

In Debian and Ubuntu CUPS is configured for a PDF-centric printing
workflow. Desktop applications send PDF, then a pdftopdf filter applies
CUPS' page management options and the result is fed into cups-pdf when
one uses my suggested architecture.

If the user cats a text file into lpr, CUPS calls texttopdf, then
pdftopdf and finally cups-pdf, again feeding PDF into cups-pdf.

Conversion of arbitrary input files is always done by CUPS and not by
cups-pdf. In the good old times of PostScript-centric CUPS cups-pdf only
needed to do the final step of turning PostScript into PDF, with the new
PDF-centric CUPS workflow this is not needed any more, CUPS delivers PDF
for us then. cups-pdf only needs to cater for special wishes, like
changing the PDF level then, and can simply save the file in most cases
as most users do not have these special wishes.

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https://bugs.launchpad.net/bugs/820820

Title:
  cups-pdf should not convert PDF print jobs to PS then to PDF

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