Downsample image-based PDF documents with Ghostscript

pdf_downsample.sh

#!/bin/bash
# Usage: pdf_downsample.sh <input_pdf_file> <output_pdf_file> [output_dpi]
# A. J. Ricoveri (github.com/axltxl)
set -e

_pdf_help() {
  echo "Usage: $0 <input_pdf_file> <output_pdf_file> [output_dpi]"
  exit 1
}

_pdf_downsample() {
  local input_pdf_file=$1
  local output_pdf_file=$2
  local output_dpi=$3
  set -x
  gs \
  -o $output_pdf_file \
  -sDEVICE=pdfwrite \
  -dDownsampleColorImages=true \
  -dDownsampleGrayImages=true \
  -dDownsampleMonoImages=true \
  -dColorImageResolution=$output_dpi \
  -dGrayImageResolution=$output_dpi \
  -dMonoImageResolution=$output_dpi \
  -dColorImageDownsampleThreshold=1.0 \
  -dGrayImageDownsampleThreshold=1.0 \
  -dMonoImageDownsampleThreshold=1.0 \
  $input_pdf_file
  set +x
}

input_pdf_file=$1
output_pdf_file=${2:-pdf_downsample.pdf}
output_dpi=${3:-300}

[ -z "$1" ] && _pdf_help
_pdf_downsample $input_pdf_file $output_pdf_file $output_dpi