PDF to Foundry isn't used to check you own the product, the creator actually wrote code that would pull every element out of the pdf and copy it into Foundry. But... it didn't need to do most of the work since Paizo publish, under and open license, all the crunch details of Pathfinder 2 and they're already in the compendium. This meant that PDF to Foundry only had to pull out maps and journal entries, but even that was so much work that the author quit. Trying to use that technique for all the information in a book would be, I suspect, untenable and certainly more work than entering it, by hand, in the first place.I'd also love to get compendiums for FoundryVTT.
I imagine that the SRDs are fair game for publicly creating (and perhaps these are already part of the twodsix system).
One other model that I've heard about is from [pdftofoundry](https://www.foundryvtt-hub.com/package/pdftofoundry/) (or perhaps another module, I don't recall offhand). The idea was that someone has entered a bunch of the data (which is no doubt copyrighted) into FoundryVTT, and that data can be injected into FoundryVTT if you can prove that you have legitimately purchased the source material from the publisher.
It's an idea that keeps knocking around in my head: that if publishers are able to somehow verify if someone should be able to use their copyrighted material, community members can potentially do the legwork of converting it to their favoured VTT, and sharing that work with others without breaking copyright.
Of course, this begs the question of if publishers are happy to do that, and also how to implement in a way that doesn't risk pirating content (at least to the publishers' satisfaction), and also preserves privacy of the publishers' customers.
I am one of the people who would happily pay - one-off payments, or a subscription - for a compendium, or compendiums of data, or just for the data itself - after which an importer could be created easily both for Foundry and, to ease the work, on Fantasy Grounds.
Mongoose aren't just losing revenue from not supporting VTTs, they're also losing the customers - like the four different groups I play with - all of whom have said they'd be really interested in Traveller, but they're not interested in hand-entering data, and they're not interested in Fantasy Grounds (which several of us purchased, and is very poor despite the best efforts of the dev, because the app itself is so poor).
Please lovely Mongoosonians, just sell or rent us data.