pdf2gray

#!/bin/bash
# Downscale colour PDF to grayscale.
# pdf2gray file1.pdf [file2.pdf] [file3.pdf] ... [filen.pdf]
#
# June 2019. yaswant.pradhan
# -----------------------------------------------------------------------------
infile=${1?:"Missing pdf file(s)"}
status=0
errfile=()
for infile in "$@"; do
  if [ ! -f "$infile" ]; then
    echo "$infile: No such file"
    errfile+=("$infile")
    ((status++))
    continue
  fi
  outfile="${infile%.*}-gray.pdf"
  printf "%s -> %s\n" "$infile" "$outfile"

  # convert to ebook format
  gs \
  -sDEVICE=pdfwrite \
  -dCompatibilityLevel=1.4 \
  -sColorConversionStrategy=Gray \
  -dProcessColorModel=/DeviceGray \
  -dPDFSETTINGS=/ebook \
  -dCompressFonts=true \
  -dNOPAUSE \
  -dQUIET \
  -dBATCH \
  -dDetectDuplicateImages \
  -sOutputFile="$outfile" \
  "$infile"
done

# report any error
if [[ "$status" -gt 0 ]]; then
  printf "\nUnable to apply conversion to the following file(s):\n"
  printf "%s\n" "${errfile[@]}" | nl
fi