Most would-be competitors to Patreon think it's some sort of DRM system. There are definitely people who try to use Patreon that way, and it works about as well as any DRM system does.
We have lots of other "pay us to access this document" platforms, starting with the 800 lb gorilla, Amazon. If you think there is some benefit to wedding a membership system to a document storefront, I think you're probably wrong. I could be convinced otherwise – since I'm not actually in that business, I assume there's a lot I don't know about it – but my guess is having to join a club just for the privilege of buying a PDF introduces friction that reduces revenue. Heaven knows I resent it as a customer.