Summary: Printing PDFs was not working (getting blank pages) and throwing up “Warning: require_once” errors.
Solution: After installing dompdf in sites/all/libraries, you need to install the php-font-lib library and then create a dompdf/lib/php-font-lib/classes/ directory, in which you put the contents of the src/FontLib/ folder.

I tried to set up the Print module on Poligo a little while ago and could not get it to work. I found this thread on Github that got me part of the way, but in the end I called it quits.

Working on The English Farm just now, I found this gem that was the missing piece.