PDF export using custom quartz filters

I wanted to reduce the size of scanned images in a PDF which shouldn't be all too hard, but all prominent search results are taken by online PDF compressors which I don't want to use, ever (beware of the clown!). TL;DR: Use a custom quartz filter, see Bessere PDF-Datei-Größenreduzierung in OS X and Mimsy: Quality reduced file size in Mac OS X Preview. Enjoy.