Is there a way to make a black and white version of the PDF with a simple GUI program?

I have a multi-page pdf with various colorful certificates that I’d want to convert to black and white (grayscale).

Is there a way to turn it grayscale with a single command or a simple GUI tool (ideally free and open source)?

In theory, I could open each page separately in GIMP, convert them to grayscale, and then reassemble them, but it would take a lot of time.

Askify Moderator Edited question April 15, 2023