How do I convert a photo collage in color to black and white?

What I meant to post before:

One approach is to open a copy of the PDF in Preview, and select File menu : Export… and apply either the Black and White, or Gray tone Quartz filter. The B & W filter will result in quite garish contrast.

The Quartz filters will not fully remove color in the PDF, even though color will not be visibly present in it when viewed in Preview again.

I have a tendency to use Ghostscript (currently 9.50) for this purpose and then test the CMYK ink coverage afterward:

gs -sDEVICE=pdfwrite -sProcessColorModel=DeviceGray -sColorConversionStrategy=Gray -dOverrideICC -o out.pdf -f in.pdf

and I follow this with an ink coverage test:

gs -o - -sDEVICE=inkcov out.pdf | grep -v Page
GPL Ghostscript 9.50 (2019-10-15)
Copyright (C) 2019 Artifex Software, Inc.  All rights reserved.
This software is supplied under the GNU AGPLv3 and comes with NO WARRANTY:
see the file COPYING for details.
Processing pages 1 through 8.
 0.00000  0.00000  0.00000  0.28453 CMYK OK
 0.00000  0.00000  0.00000  0.07611 CMYK OK
 0.00000  0.00000  0.00000  0.32368 CMYK OK
 0.00000  0.00000  0.00000  0.18324 CMYK OK
 0.00000  0.00000  0.00000  0.08770 CMYK OK
 0.00000  0.00000  0.00000  0.08835 CMYK OK
 0.00000  0.00000  0.00000  0.09274 CMYK OK
 0.00000  0.00000  0.00000  0.03931 CMYK OK

Since the first three color-bearing columns (Cyan, Magenta, and Yellow) are zero, this tells me that no color exists in the PDF.

Sure, though in GC 11, it appears as grayscale when opened as such, when saved, Ghostscript ink coverage shows that the saved GC 11 PDF to have CMY colors present. Most people will not either know this, or care, but some want no color in the PDF at all.

One approach is to open a copy of the PDF in Preview, and select File menu : Export… and apply either the Black and White, or Gray tone Quartz filter.

If you are printing locally you can open the PDF: command P to open the Print Dialogue>Presets> Black and White

Print documents from your Mac - Apple Support

That's easier. One best use the Grey Tone filter I think…

Original…

B&W Filter

Grey filter…

If I had to do it I would export the PDF as a .png, open the ,png and use "Adjust Color" to remove the saturation. That may or may not suffice for your need.

I must be tired. This would be done in Preview.

FWIW, you can also use GraphicConverter 11 which can convert a color PDF to greyscale on opening.