Make PDF files smaller

1. Single PDF
2. PDFs in a folder

The most effective ways of reducing the file size of a PDF is by converting to grayscale and reduce the resolution.

Requirement: ghostscript (installed by default in Ubuntu)

Single PDF §

Save the following script as “pdfcompress.sh”.

Usage: sh pdfcompress.sh input.pdf

The output compressed file is named as “input.compressed.pdf”

#!/bin/sh

filename=$(basename "$1")
dir=$(dirname "$1")

gs \
  -sOutputFile="$dir/${filename%.*}.compressed.pdf" \
  -sDEVICE=pdfwrite \
  -dPDFSETTINGS=/ebook \
  -sColorConversionStrategy=Gray \
  -sColorConversionStrategyForImages=/Gray \
  -dProcessColorModel=/DeviceGray \
  -dCompatibilityLevel=1.4 \
  -dNOPAUSE -dBATCH -dQUIET \
  "$1"Copy

Options (more info):

• Remove ColorConversionStrategy, ColorConversionStrategyForImages and ProcessColorModel lines to retain colour.
• PDFSETTINGS:
  • /default selects output intended to be useful across a wide variety of uses. 72 DPI.
  • /screen selects low-resolution output similar to the Acrobat Distiller “Screen Optimized” setting. 72 DPI.
  • /ebook selects medium-resolution output similar to the Acrobat Distiller “eBook” setting. 150 DPI.
  • /printer selects output similar to the Acrobat Distiller “Print Optimized” setting. 300 DPI.
  • /prepress selects output similar to Acrobat Distiller “Prepress Optimized” setting. 300 DPI.

PDFs in a folder §

Use the following script to compress all PDFs in a folder.

Usage: sh pdfcompress.sh 'target folder'

If 'target folder' is not specified, defaults to current directory.

#!/bin/sh

if [ -n "$1" ]; then
  cd "$1"
fi

for i in *.pdf; do
    [ -f "$i" ] || break

  # Skip compressed PDFs
  echo "$i" | grep --quiet ".compressed.pdf"

  if [ $? = 1 ]; then
    gs \
      -sOutputFile="${i%.*}.compressed.pdf" \
      -sDEVICE=pdfwrite \
      -dPDFSETTINGS=/ebook \
      -sColorConversionStrategy=Gray \
      -sColorConversionStrategyForImages=/Gray \
      -dProcessColorModel=/DeviceGray \
      -dCompatibilityLevel=1.4 \
      -dNOPAUSE -dBATCH -dQUIET \
      "$i"
  fi
doneCopy

If you prefer to use the original filename as the compressed version and rename the original uncompressed’s to “*.original.pdf”,

Following script doesn’t skip previously compressed PDFs

#!/bin/sh

if [ -n "$1" ]; then
  cd "$1"
fi

for i in *.pdf; do
  [ -f "$i" ] || break

  # Rename original file to *.original.pdf
  original="${i%.*}.original.pdf"
  mv "$i" "$original"

  gs \
    -sOutputFile="$i" \
    -sDEVICE=pdfwrite \
    -dPDFSETTINGS=/ebook \
    -sColorConversionStrategy=Gray \
    -sColorConversionStrategyForImages=/Gray \
    -dProcessColorModel=/DeviceGray \
    -dCompatibilityLevel=1.4 \
    -dNOPAUSE -dBATCH -dQUIET \
    "$original"
doneCopy

Source: Internal Pointers, firstdoit, ahmed-musallam, Ghostscript Docs