As this says: "METHOD: SLOWEST, MORE COMPRESSION". Unfortunately slowest and more compression are relative terms. However given the variability in file compression that is truthfully about all anyone can say with a straight face.

My point was that it appears that like graphics files, PDFs are not amenable to file compression. PDFs can be compressed but, like graphics compression, it is a lossy process that works by discarding hopefully extraneous data. Whereas zip/7zip/rar/et. al. take great pains not to discard even a single bit of data.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein