Depends ok the DRM technology. The only PDF DRM I came across so far was using the horrible feature by adobe digital editions.

It can be removed by using calibre and an unofficial plugin called noDRM, which should work on Mac as well (didnt test it tho, I dont own a Mac. Worked fine on windows tho)

Dream Theater and Classical music have entered the chat

Aegis authenticator. Beats all proprietary apps I’ve tried so far

I’ve been contributing to it via Street complete (can get it on fdroid) and it has become quite a hobby of mine.