What settings to use when making 7zip files in order to get maximum compression when compressing PDFs?

Try precomp - it first decompresses the already compressed data inside of your PDFs. Then 7z can do its magic on uncompressed data.

Also try nanozip which I have verified to be very effective, yet very efficient (400kb/s at compression ratios of PAQ algorithms).

The content of the PDFs (text & images) is probably already compressed -- so there's not going to be much to gain by trying to compress them again.

7za a -t7z -mx-9 -mfb=258 -mpass=15 filename.7z subdir

Adjust the first word as necessary for the name of your command line executable, and adjust the parts after "-mpass=15" to customize your filename and what it should include.

This answer is not specific to PDF documents.
This uses LZMA, not PPM. I've stayed away from PPM because there are too many variations that are not compatible with other variations. LZMA looks to be more stable, with compatibility being more widely supported. So I've stayed away from PPM precisely because my opinion was, as you've stated, "the effort needed to use exotic compression algorithms isn't justified."