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I recommended this for Quoth's archive.org download (with substantially yellowed pages):
k2pdfopt -mode copy -dpi 400 -c- -g 1 -bpc 4 -cmax -5 sourcefile.pdf
-dpi 400 for 400 dpi. The source file has a dpi of 600.
-c- to turn off color (make it grayscale)
-g 1 to turn off gamma correction (default of -g 0.5 makes things darker)
-bpc 4 to get 4-bit grey scale (the default)
-cmax -5 to force the contrast level to 5 to get the page background mostly white.
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