Historically, AFP compressed far better than PDF. That may change in the newer versions of CMOD, which can 'decompose' PDFs (aka remove bulky resources like images and fonts), then 'recompose' them on retrieval -- in exactly the same way CMOD does for AFP data.
The requisite for this is the PDF indexer, which is a separately licensed product or 'bolt-on-feature', which makes the cost of the separate software another variable...
Good luck!
-JD.