Gimp can open PDFs, if you select “open pages as images” instead of the default “as layers”, it will open each page as a separate image.

Then you can use burn/levels/… to improve quality of the scan of the document printed with a printer that’s low on toner.

Also - Goddammit Gimp interface - was looking for the burn tool. It’s hidden behind “Smudge”, had to use right click on it to get the full list. Hate this