Depending on the pdf creator/engine used, a .pdf file may include content irrelevant for reading by a human. An example is the inclusion of complete sets of fonts though only a few glyphs are (or only one is) used on the "electronic paper". To print the .pdf with ghostscript again into a .pdf may reduce the file size.

By far, this bash script does not claim to be the first one collecting bits and bolts to address the issue. It rather serves as an aide-memoire of finds encountered earlier, and to moderate ghostscript in Linux accordingly. Within reason, the snippets were joined as provided; thus, the credit belongs to those already in the field.

The most comfortable use is 1) the provision of the executable bit (chmod +x pdf_reprint.sh), and 2) to set an alias in your .bashrc file. Then, its functionality described below is available all across your system.

• Else, to reprint the .pdf while retaining the color, run either one of the following commands

  bash ./pdf_rewrite.sh --reprint input.pdf
  bash ./pdf_rewrite.sh -r input.pdf
  bash ./pdf_rewrite.sh --colour input.pdf
  bash ./pdf_rewrite.sh --color input.pdf
  bash ./pdf_rewrite.sh -c input.pdf

  to replace the original file input.pdf by a new version of smaller file size. There will be a short note if the attempt was successful; and if so, the improvement compared to the original input.pdf is reported (percentage). Though you may process the file multiple times with this method, savings in file size often quickly converge to be insignificant in comparison to the remaining file size.

  The credit for the underlying approach and implementation belongs to Evan Langlois.¹

• Often, an additional reduction of file size may be obtained by reprinting the .pdf in gray-scale only. Either one of the following commands to the script is equivalent

  bash ./pdf_rewrite.sh --gray input.pdf
  bash ./pdf_rewrite.sh --grey input.pdf
  bash ./pdf_rewrite.sh -g input.pdf

  to replace file input.pdf by its rewritten form. The credit for this approach belongs to user slm on the Unix stackexchange.²

• To process multiple .pdf, a for-loop in your shell could follow the pattern of

  for file in *.pdf
  do
      echo "$file"
      bash ./pdf_rewrite.sh -r "$file"
  done

  This equally provides you a brief progress report, too.

Note, this script's primary aim is to obtain a file of small file size, e.g., as an attachment of an email while retaining the text easy to read and – if present – to retain a text layer searchable. Especially the reprint in half-tones however may render illustrations less intelligible. It is up to the creators of figures to use easy discernible markers in diagrams, as well as to use color scales suitable for the color blind, and safe for this mimicked "photocopying". For the later, a service like https://colorbrewer2.org/ may guide your selection.

Keep a backup of the .pdf to be processed. Though the script may report problems while processing the data (or even crash, which may destroy the .pdf), it is not a PDF validator such as e.g., veraPDF.³

Initially written for Linux Xubuntu 18.04.3 LTS and ghostscript (version 9.26), the archive includes two .pdf files to check the scripts action e.g., with Debian 12/bookworm (currently testing) and ghostscript (version 10.00.0).

• File link2web.pdf was compiled with pdfLaTeX based on an example provided by www.texample.net. This .pdf contains a color figure and link to an external reference. Note, the simplification into half-tones (option -g) affects the document printed, depending on the pdf viewer used, the box around the link may remain colored for the display on screen.

• File Kletskov2020.pdf is a contemporay research article⁴ including text, colour line art, and photographs which was published under a permissive Creative Commons license.

In the table below, savings computes the difference of the file size prior and after the processing with either option, then reports this change as percentage in respect to the original file size.