Compress PDFs for electronic faxing, where the attachment size limits are very small, by making them monochrome, pixelated, 200 DPI. Parallelized.

fax-convert.sh

#!/bin/zsh
# brew install qpdf parallel imagemagick

set -e

tmpdir=$(mktemp -d)
trap 'rm -rf -- "$tmpdir"' EXIT

echo "Splitting PDF"
qpdf --warning-exit-0 --no-warn --split-pages=5 "$1" "$tmpdir/%d-out.pdf" # five page chunks

echo "Converting pages"
ls "$tmpdir"/*out.pdf | parallel --progress -j0 magick -monochrome -density 200 {} -compress Group4 {.}-comp.pdf

echo "Combining PDF"
qpdf --empty --pages "$tmpdir"/*-out-comp.pdf -- "${1%.*}_faxcompress.${1##*.}"
rm -rf -- "$tmpdir"