Much of the back catalogue is already available open access direct from the Royal Society:

Most of our oldest content is now freely available, specifically, all papers older than 70 years. In addition, papers published between 10 years ago and either 12 months ago (biological sciences) or 24 months ago (physical sciences) are freely available. For Biographical Memoirs all issues are now freely available, apart from the most recent issue.

That's odd. What about the papers published between 70 and 10 years ago?

Ancient organisations like the Royal Society have arcane rules and oddities. This is likely one of them.

Sci-hub is already available, no need to send zip files around for distribution