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.
I just saved this page as a PDF, and zipping it resulted in a 72% saving, whereas zipping an assortment of "graphics rich" PDFs resulted in savings of between 3 & 9%, so
I guess as you suggested, the amount of empty space in a given PDF is critical.
