Compressing a PDF using Ghostscript on command line

pdf-compress.sh

# utility functions
#

# compress PDF using ghostscript
#
# https://ghostscript.readthedocs.io/en/latest/VectorDevices.html#controls-and-features-specific-to-postscript-and-pdf-input
#
# -dPDFSETTINGS=/screen lower quality, smaller size. (72 dpi)
# -dPDFSETTINGS=/ebook for better quality, but slightly larger pdfs. (150 dpi)
# -dPDFSETTINGS=/prepress output similar to Acrobat Distiller "Prepress Optimized" setting (300 dpi)
# -dPDFSETTINGS=/printer selects output similar to the Acrobat Distiller "Print Optimized" setting (300 dpi)
# -dPDFSETTINGS=/default selects output intended to be useful across a wide variety of uses,
#   possibly at the expense of a larger output file
#
# Usage: pdf-compress sm|md|lg /path/to/input.pdf /path/to/output.pdf
#
function pdf-compress {
  # Check for required arguments
  if [ $# -lt 2 ]; then
    echo "Usage: pdf-compress sm|md|lg input.pdf [output.pdf | input-<size>.pdf]"
    return 1
  fi

  # Validate size argument
  if [[ ! "$1" =~ ^(sm|md|lg)$ ]]; then # Improved pattern for exact match
    echo "Please use sm, md, or lg. *$1* is not a valid size."
    echo "Usage: pdf-compress sm|md|lg input.pdf [output.pdf]"
    return 1
  fi

  # Determine size based on argument
  # default = medium size compress
  size="ebook"
  # check which size we need
  case "$1" in
  "sm") size="screen" ;;
  "md") size="ebook" ;;
  "lg") size="prepress" ;;
  esac

  # Check if gs command exists
  if ! command -v gs &>/dev/null; then
    echo "Error: Ghostscript (gs) is required. Please install it."
    return 1
  fi

  # Construct output filename
  if [ $# -eq 3 ]; then
    filename="$3"
  else
    # Efficiently create "input-size.pdf" format
    filename="${2%.*}-${size}.pdf"
  fi

  # Execute compression using gs
  gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS="/$size" \
    -dNOPAUSE -dQUIET -dBATCH -sOutputFile="$filename" "$2"

  # Optional: Open the compressed PDF (comment out if not needed)
  open -R "$filename"

  # Optional: show success message
  # echo "PDF compressed successfully: $filename"
}