For some applications it is necessary to keep the file size of an PDF under a certain limit. For example for  document uploads on some websites or to maintain the attachment file size limit of some email server. 

You can use the following shell script to have a schrink command (Source: https://dev.to/manishfoodtechs/shrinking-file-size-of-pdf-using-a-small-bash-script-2pd1).

#!/bin/sh
 
shrink ()
{
	gs					\
	  -q -dNOPAUSE -dBATCH -dSAFER		\
	  -sDEVICE=pdfwrite			\
	  -dCompatibilityLevel=1.3		\
	  -dPDFSETTINGS=/screen			\
	  -dEmbedAllFonts=true			\
	  -dSubsetFonts=true			\
	  -dColorImageDownsampleType=/Bicubic	\
	  -dColorImageResolution=150		\
	  -dGrayImageDownsampleType=/Bicubic	\
	  -dGrayImageResolution=150		\
	  -dMonoImageDownsampleType=/Bicubic	\
	  -dMonoImageResolution=150		\
	  -sOutputFile="$2"			\
	  "$1"
}
 
check_smaller ()
{
	# If $1 and $2 are regular files, we can compare file sizes to
	# see if we succeeded in shrinking. If not, we copy $1 over $2:
	if [ ! -f "$1" -o ! -f "$2" ]; then
		return 0;
	fi
	ISIZE="$(wc -c "$1" | cut -f1 -d
)"
	OSIZE="$(wc -c "$2" | cut -f1 -d
)"
	if [ "$ISIZE" -lt "$OSIZE" ]; then
		echo "Input smaller than output, doing straight copy" >&2
		cp "$1" "$2"
	fi
}
 
usage ()
{
	echo "Reduces PDF filesize by lossy recompressing with Ghostscript."
	echo "Not guaranteed to succeed, but usually works."
	echo "  Usage: $1 infile [outfile]"
}
 
IFILE="$1"
 
# Need an input file:
if [ -z "$IFILE" ]; then
	usage "$0"
	exit 1
fi
 
# Output filename defaults to "-" (stdout) unless given:
if [ ! -z "$2" ]; then
	OFILE="$2"
else
	OFILE="-"
fi

Usage

$> shrink "$IFILE" "$OFILE" || exit $?
 
$> check_smaller "$IFILE" "$OFILE"

Webservice to shrink PDF

You can also use one of the following webservices to reduce the file size of a PDF.

• https://docupub.de/pdfcompress/
• https://www.adobe.com/de/acrobat/online/compress-pdf.html
• https://tools.pdf24.org/en/compress-pdf

Other possibilities

There are other possibilities to shrink PDFs. 

• Adobe Acrobat:
  • https://helpx.adobe.com/acrobat/how-to/compress-pdf.html
  • https://helpx.adobe.com/acrobat/using/optimizing-pdfs-acrobat-pro.html
• Sheep PDF: https://www.pdf24.org/de/