Converting a PDF to greyscale

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.

As it turns out, Affinity Publisher 2 can correctly import the troublesome PDFs as grey scale then export them.

dialabrain wrote:

Apparently only a few PDF's don't convert correctly. No idea why.

PDF is just horribly complicated. I was able to find one that demonstrated the same problem. It isn't related to the first page. Any page could fail to convert. My guess is that these are deeply embedded images and Apple's filter doesn't affect them. PDFs can embed entire PDFs inside.

I even did a test. I took the page that didn't convert, copied it out as a TIF, then pasted it back it. Then it converted correctly.

dialabrain wrote:

As it turns out, Affinity Publisher 2 can correctly import the troublesome PDFs as grey scale then export them.

But the OP specifically said they didn't want to use Acrobat Pro because it's too expensive. I don't know the details of the OP's requirements. Do they just want to convert one file? Or is this an ongoing need? A single month of Acrobat Pro would be cheaper than Affinity Publisher.

In some cases, money can make problems go away. But when so few people are willing to spend, eventually, the price and difficulty skyrocket. Everybody loses in the end.

VikingOSX wrote:

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

FWIW, that may be the case but it doesn't explain why with some PDFs all the pages except one appear grey.

Using ink_cov still shows CMY color channels with non-zero values when exporting a PDF from Preview with the gray tone Quartz filter. But who cares…?

The average user that is not sending PDFs to commercial printers and just wanting apparent grayscale will likely not care about color lurking behind the grayscale from Preview. Commercial printers may expect PDF/X-3 content and that rules out Apple's PDF framework generation capabilities or Quartz filter (even though a ColorSync Quartz filter exists).

I think that’s the wrong device. The “inkcov” device reports the fraction of pixels having a given colour. You want the “ink_cov” device instead.

Not that I necessarily trust Ghostscript anyway. PDF is a mess. It just happens to be ubiquitous.

Also, it isn’t well-defined what “gray” even means. Are we talking about a PDF that looks gray or a PDF that print on a CMYK device using only K? Is that even possible? Or does it depend on a real device? I wouldn’t want to try to create a gray with CMYK halftones. But on a laser printer, I might prefer to do that.

amp68 wrote:

I'm having the opposite problem with my PDF. It converts only the first page.

FWIW, it 's become rather obvious one cannot be sure a PDF can be converted to grey scale using Preview.

You can send Apple feedback if you like.

Feedback - macOS - Apple

You may try to fix this using Python: https://stackoverflow.com/questions/62504730/how-to-convert-a-pdf-to-grayscale-in-python

I believe the result will be the same, but you could give it a try.

Cheers,

Leandro

VikingOSX wrote:

I cannot explain why you encountered an omitted page in the gray tone export process with Preview. Cannot duplicate it here on Sonoma 14.1.1.

I know. It only happens with certain PDFs. Not something I ever do in any case.

amp68 wrote:

I'm having the opposite problem with my PDF. It converts only the first page.

I'm sorry. I really thought this would be an easy answer. My biggest worry when I typed out my first reply was that someone else would type faster and get the first reply.

It seems that, more and more, every answer eventually devolves into some "it's complicated" thread like this one. I really wanted to be able to provide a simple, step-by-step reply that I know would work. I had no idea that the very next file I tested wouldn't work.

I guess I should have known. Apple Preview has been a mess for years. Having to be a cynic every day is exhausting. Just once, I wanted to be able to solve a problem. I'm very confident that, ten year ago, it would have worked perfectly. I'm going to try it in a 10.13 VM to test. I know that I have apps that can do all kinds of PDF manipulations. But of course, they don't work on modern versions of macOS. Free apps like Apple Preview, that do 60% of what you need, or free apps like Ghostscript, that do 100%, but require advanced technical skills, have killed the market for many apps, especially for the Mac.

I don't know of any easy answer.

Adobe Acrobat Pro has a Print Production -> Preflight > Convert to Grayscale feature, but who wants that $2nn annual subscription fee or the $5nn cost of the single purchase price for Acrobat Pro 2020.

It would appear that Master PDF Editor (v5.9.70) can only print, but not export, to Grayscale (e.g.), and the current Nitro PDF Pro (v13.3.1) uses the export Apple Quartz Gray Tone filter for Grayscale.

Interesting. I tried the same thing on two different Macs, running Sonoma & Monterey with a 58 page PDF. It converted all but the first page to Grey Tone.

Apparently only a few PDF's don't convert correctly. No idea why.