In theory, that means that anybody can create a fully compatible editor/reader, and if Adobe tries to change theirs it will be them that are incompatible.

In practice, I don't know why people are talking about PDF. In any recent time, I have only seen people using Adobe readers by accident and haven't heard about anybody using their editor for any reason. I know of some people that buy their editors, just not any that use it.

It's been open and free on Adobe's website for 20+ years, aside from proprietary extensions like XFA. It's the ISO standards that until recently required payment (as that's how ISO generally works).