Both filters call Ghostscript to convert PDF into PostScript. The
original one (binary) calls Ghostscript with the "pswrite" output device
and the new one (script) calls Ghostscript with the "ps2write" output
device. The "pswrite" output device turns all text characters into
bitmaps whereas the "ps2write" conserves the fonts. The conversion of
the characters into bitmaps causes several problems, the squares when
the data is fed into the "pxlmono" driver (this bug and the Ghostscript
bug #690025 linked above), large output files (bug 44989), and corrupted
characters on PostScript printers (bug 362186).

So principally one should prefer "pswrite" over "ps2write", but
"ps2write" has also a disadvantage: The output is not DSC-compliant, so
page manipulation utilities (like CUPS' pstops filter) will not fully
work with these files. In our case it works well, as page m,anipulation
was done already before by the pdftopdf filter. After the pdftops filter
we only embed Postscript code for the option settings into the job, and
this does not need DSC compliance.

-- 
black squares appearing instead of some letters when printing
https://bugs.launchpad.net/bugs/361772
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