Seems like Airbnb could monitor certificate transparency logs for these sketchy domains to inspect and initiate legal action against (at least for domains using not using an airbnb subdomain).
It's free and cross-platform with collaboration and annotation features ... yep, Papers was a waste of money (Papers did finally fix the bug, so it starts without crashing now that I don't use it).
On the other hand signing up created a public social network profile, subscribed me to a newsletter and updates, and seems to offer no way to delete the account. Great, I just wanted to play with the software.