How to compress PDF with ghostscript

compress_pdf.md

How to compress PDF using ghostscript

As a developer, it bothers me when someone sends me a large pdf file compared to the number of pages. Recently, I recieved a 12MB scanned document for just one letter-sized page... so I got to googlin, like I usually do, and found ghostscript!

to learn more abot ghostscript (gs): https://www.ghostscript.com/

What we are interested in, is the gs command line tool, which provides many options for manipulating PDF, but we are interested in compressign those large PDF's into small yet legible documents.

credit goes to this answer on askubuntu forum: https://askubuntu.com/questions/3382/reduce-filesize-of-a-scanned-pdf/3387#3387?newreg=bceddef8bc334e5b88bbfd17a6e7c4f9

Steps below were only tried on macOs sierra

you can install gs via the official site or via homebrew

brew install ghostscript

now to compress a pdf:

gs 
 -q -dNOPAUSE -dBATCH -dSAFER \
 -sDEVICE=pdfwrite \
 -dCompatibilityLevel=1.3 \
 -dPDFSETTINGS=/screen \
 -dEmbedAllFonts=true -dSubsetFonts=true \
 -dColorImageDownsampleType=/Bicubic \
 -dColorImageResolution=144 \                `#PDF downsample color image resolution`
 -dGrayImageDownsampleType=/Bicubic \
 -dGrayImageResolution=144 \                 `#PDF downsample gray image resolution`
 -dMonoImageDownsampleType=/Bicubic \
 -dMonoImageResolution=144 \                 `#PDF downsample mono image resolution`
 -sOutputFile=out.pdf \                      `#Output file`
 file.pdf                                    `#Input file`

you can find documentation on ghostcript commands here: https://www.ghostscript.com/doc/current/Use.htm#Options

you'll notice that I set all the ImageResolution options to 144, I found that this value gives the best results for legible text scans, you can change that to whatever you like

I also added a function to my .bash_profile to make a shorthand that will compress and rename file.pdf to file.pdf.compressed.pdf:

pdfcompress ()
{
   gs -q -dNOPAUSE -dBATCH -dSAFER -sDEVICE=pdfwrite -dCompatibilityLevel=1.3 -dPDFSETTINGS=/screen -dEmbedAllFonts=true -dSubsetFonts=true -dColorImageDownsampleType=/Bicubic -dColorImageResolution=144 -dGrayImageDownsampleType=/Bicubic -dGrayImageResolution=144 -dMonoImageDownsampleType=/Bicubic -dMonoImageResolution=144 -sOutputFile=$1.compressed.pdf $1; 
}

use it: pdfcompress somefile.pdf

I believe the setting for the monochrome downsampling type to Bicupic doesn’t contribute to the overall compression or quality of the pdf. See this quote from the Ghostscript blog (:

Moving on, the final parameter is the type of downsampling to apply, and there are three possibilities: Subsample, Average and Bicubic. For Monochrome images, we can only use Subsample because the other types involve coming up with some kind of average value of the pixels we are considering. Since monochrome images can only have black or white pixels, there’s no way to come up with an average.

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