make an image look like it was scanned

scan_pdf.sh

# separate pdf pages
convert -density 400 -colorspace gray doc.pdf page-%0d.png

# alternative if convert fails (result does not look the same as original)
# pdftoppm -gray doc.pdf page -png

# use gimp to insert the signature at the right place at the right page, remove unnecessary pages etc

# scan
for F in page*png; do
  convert -resize 1800x1800 -density 95 -flatten "$F" -rotate "0.$(( ( RANDOM % 2 )  + 3 ))" +repage -attenuate "0.$(( ( RANDOM % 8 )  + 20 ))" +noise Gaussian  -quality 85 -compress JPEG "$F-scanned.jpg"
done

# combine the pages into a pdf
convert *scanned.jpg scan.pdf