Script (or some other means) to convert RGB to CMYK in PDF?

Question

Is it possible to write a script for Adobe Illustrator or some other tool that will read the contents of a number of PDF files and convert all the RGB colours to CMYK?

If so, could somebody please point out some of the steps involved, or where to find more information on this?

Answer 1

This answer is not for Illustrator, but for 'some other tool', namely Ghostscript (download gs871w32.exe or gs871w64.exe).

Ghostscript allows you to 're-distill' PDFs (without an intermediate conversion to PostScript, the dreaded 'refrying' detour). Try this command:

gswin32c.exe ^
    -o c:/path/to/output-cmyk.pdf ^
    -sDEVICE=pdfwrite ^
    -dUseCIEColor ^
    -sProcessColorModel=DeviceCMYK ^
    -sColorConversionStrategy=CMYK ^
    -sColorConversionStrategyForImages=CMYK ^
     input-rgb.pdf

And if you are able to wait for a few more weeks, Ghostscript 9.00 will be released. This new version will sport support of colormanagement (based on LCMS) with ICC profiles for the first time ever...

UPDATE: I updated above command because I missed to put in the option to also convert images.

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Update 2

If color conversion does not work as desired and if you see a message like "Unable to convert color space to Gray, reverting strategy to LeaveColorUnchanged" then...

1. your Ghostscript probably is a newer release from the 9.x version series, and
2. your source PDF likely uses an embedded ICC color profile

In this case add -dOverrideICC to the command line and see if it changes the result as desired.