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ImageMagik
Loads of other tools
Though for security reasons I open the PDF in The Gimp at a required resolution. Each page is a layer. I then save as/Export as motion png in reverse order so the pages are correct. Then imagemagik can crop, darken text, brighten background, change from RGB to greyscale or 16 shades grey or 1 bit mono by reading mpng file and creating a pdf file.
Then the PDF pages turn about x10 faster too.
Some PDF readers can do this on the fly, but not the stock one on Kindle or Kobo.
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