[kbmn]

I had success with the famous scanner.sh gist (https://gist.github.com/andyrbell/25c8632e15d17c83a54602f6ac...):

  $ convert -density 90 input.pdf -rotate 0.5 -attenuate 0.2 +noise Multiplicative -colorspace Gray output.pdf

(convert command -> requires ImageMagick)