reduce_pdf_dpi.sh

#!/bin/bash

shopt -s dotglob

# use bash strict mode
set -euo pipefail
IFS=$'\n\t'

DPI="${DPI:-120}"

if [[ "$#" != "2" ]]; then
    echo "usage: $0 input.pdf output.pdf"
    exit 2
fi

input="$1"
output="$2"

"gs" -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH \
    "-sOutputFile=$output" \
    "-dColorImageResolution=$DPI" "-dGrayImageResolution=$DPI" "-dMonoImageResolution=$DPI" "$input"