How to reduce PDF file size on Macs and Linux?

 Of the suggestions I saw, using the "shrink PDF" script files with Ghostscript was the best.

Install Ghostscript first.  On Linux, apt install ghostscript or on Macs using homebrew, brew install ghostscript.

Save that shrink PDF script into a shrinkpdf.sh file.  Make sure to chmod u+x shrinkpdf.sh to allow it to execute.

Then use it, /path/to/shrinkpdf.sh in.pdf out.pdf.

On my old Mac running Sierra macOS 10.12, Ghostscript had trouble running because the way brew built it, Ghostscript was looking for the libXt.6.dylib library in the wrong place.

You need XQuartz installed first.  But XQuartz installs X11 libraries into /opt/X11.  But Ghostscript looks for X11 libraries in /usr/X11.

Easy fix?  Symlink the X11 directory to where Ghostscript expects it.

Go to where Ghostscript looks for X11: cd /usr

Then symlink to where the X11 libraries actually are: sudo ln -s /opt/X11.

Done.