User profile for user: amp68
amp68 Author
User level: Level 1
84 points

Converting a PDF to greyscale

I am trying to convert a 212 page PDF file from color to greyscale.


I first attempted to Print, turn off print in color, and save as PDF. But that doesn't work, even though a half dozen websites say it should. The resulting PDF is still in color.


Next I tried another to use the Export feature of Preview and apply the Gray Tone Quartz filter. That only converts the first page of the PDF to greyscale. The rest stay color.


I next tried to use the ColorSync Utility, as Apple's KB advises. That also only makes the first page greyscale and the last PDF in color.


Is there some way I can make this happen on the Mac and preserve the text layer.


Things I WILL NOT do:


  1. I will not upload to some website for conversion.
  2. I will not use Acrobat Pro. It's too expensive.

Mac mini, macOS 13.2

Posted on Nov 22, 2023 1:56 PM

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User profile for user: VikingOSX
VikingOSX
User level: Level 10
130,094 points

Posted on Nov 22, 2023 4:56 PM

Preview is incapable of exporting a truly grayscale PDF using Quartz Gray tone. The result is still RGB even if it does appear gray.


I happen to have Ghostscript 10.02.1 installed (via homebrew) and it does generate a C:0 M:0 Y:0 K:* output pdf. I can verify that using its ink coverage device and a graytone.pdf exported using Quartz Gray Tone.


gs -o - -sDEVICE=inkcov graytone.pdf
Page 1
0.01204  0.03094  0.01216  0.01105 CMYK OK
Page 2
0.02645  0.04471  0.02645  0.02645 CMYK OK
︙
Page 288
0.01875  0.35909  0.01875  0.01875 CMYK OK


Inkcov is reporting the fraction of ink coverage (converted from RGB) for each color channel in the CMYK space.


Same PDF as opened in Preview, but processed by Ghostscript:

gs \
-sDEVICE=pdfwrite \
-sProcessColorModel=DeviceGray \
-sColorConversionStrategy=Gray \
-dOverrideICC \
-o graytone.pdf -f hp15c.pdf


and when I run inkcov on graytone.pdf:


gs -o -  -sDEVICE=inkcov graytone.pdf
Page 1
0.00000  0.00000  0.00000  0.01141 CMYK OK
Page 2
0.00000  0.00000  0.00000  0.02636 CMYK OK
︙
Page 288
0.00000  0.00000  0.00000  0.09498 CMYK OK


With ghostscript, you can see that the CMY channel values are always zero with ink only being measured in the K (black) channel throughout each page of the document.

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Converting a PDF to greyscale

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