I have a shell script based on ImageMagick that gives a PDF a "scanner" look. I typically open the PDF in Master PDF Editor to insert an image of my signature, then pass it through my script. When I do need it, it's rare, but it becomes a real life saver. It has avoided me the need to print and scan 100+ pages for a mortgage company, some stock brokers and banks. Key points of the script:

"+noise Random -fill white -colorize 95%" to add some noise to the image

"-distort ScaleRotateTranslate '$x,$y $angle'" to randomly shift horizontally and vertically the document, and randomly rotate it slightly

"-density 150" for a low-ish resolution so it better hides the fact the PDF wasn't really scanned

"-colorspace Gray" to make it black & white

"-quality 60" to increase JPG compression and somewhat reduce picture quality

  #!/bin/bash
  # Make a pdf look like it was scanned.
 
  if [ $# -ne 2 ]; then
      echo "Usage: $0 input output" >&2
      exit 1
  fi
  tmp="$1".scanner-look.tmp
  mkdir "$tmp" &&
  # without -flatten some PDF convert to a JPG with a black background
  convert -density 150 "$1" -colorspace Gray -quality 60 -flatten "$tmp"/p_in.jpg &&
  : || exit 1
  # each page is randomly shifted in the X and Y plane.
  # units seem to depend on angle of rotation in ScaleRotateTranslate?
  offset() { echo $(($RANDOM % 1000)); }
  for f in "$tmp"/p_in*jpg; do
      # each page is randomly rotated by [-0.5 .. 0.5[ degrees
      angle=$(python -c 'import random; print(random.random()-0.5)')
      x=$(offset)
      y=$(offset)
      convert "$f" \
        -blur 0x0.5 \
          -distort ScaleRotateTranslate "$x,$y $angle" +repage \
        \( +clone +noise Random -fill white -colorize 95% \) \
        -compose darken \
        -composite \
        ${f/p_in/p_out}.pdf || exit 1
  done
  # concatenate all the pages to one PDF
  # use "ls -v" to order files correctly (p_out-X.jpg where X is 0 1 2 ... 9 10 11 ...)
  pdftk $(ls -v "$tmp"/p_out*.pdf) cat output "$2" &&
  rm -rf "$tmp"