Best way to convert color PDFs to grayscale on Windows 10

January 6, 2022 1:09 PM

I have about 5,000 PDFs that were created through a desktop scanner. They're all in color. I would like to convert them to grayscale but without any other loss of quality.

I'm hoping for some kind of batch process to do this (I'm certainly not going to convert them individually). I don't have access to Adobe Acrobat Pro or any other Adobe products. Googling leads me to some dodgy-looking utilities that cost money. A few forum posts (like this one) say that Ghostscript used to be able to do this, but it doesn't work anymore.

Open source would be best – but I'm not averse to paying some money for software that will accomplish this task, even though I don't anticipate the need to do this ever again. I'm running Windows 10.

posted by akk2014 to Computers & Internet (3 answers total)

On Windows, GIMP or ImageMagick are the workhorses. They can be batched or scripted.
posted by dum spiro spero at 1:29 PM on January 6, 2022 [2 favorites]

My job has been to do sort of thing to image files by millions and LibTiff and ImageMagick, run via batch files on the command line, were our workhorses for making things look the way we wanted.
posted by AzraelBrown at 1:59 PM on January 6, 2022

do you need to convert them to true black and white (every pixel is either pure black or pure white) or greyscale? my read of the discussion in that superuser.com question is that only conversion to true black and white no longer works how it used to.

I just tried this command in GhostScript 9.55.0, lifted from one of the answers in that thread:

gs -sOutputFile=outputfilename.pdf -sDEVICE=pdfwrite -sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray -dCompatibilityLevel=1.4 input.pdf

on some random PDFs I have lying around, and it correctly removed the colour and returned a greyscale PDF.
posted by russm at 2:45 PM on January 6, 2022 [2 favorites]