You can embed arbitrary javascript in PDFs

This doesn't seem usefully true. Some googling for search queries like 'a/b testing PDFs' or 'PDF phone home' show no one discussing the topic of A/B testing different versions of PDFs, and Wikipedia indicates that only Adobe supports JS and even it produces a popup when you try to phonehome. So any A/B test is going to work on only a fraction of users (how many LWers still use Adobe Acrobat to read PDFs?), and it will alarm the ones it does work on ('is this PDF spyware‽').