There's only really two organisations I know of who actively try and dig down into this stuff and give it back to the public. (There are a lot who just accept the legal risks and ignore what rules there may be.)

The Internet Archive [0], and TorrentFreak[1], with the former being the more active in the area.

My own knowledge comes from more selfish, commercial, reasons, in that I've tried to use PD content to kickstart my site's collection. (To be clear: Not affiliated with the parent of this thread in any way shape or form.)

If you want to donate - I'd highly recommend the Internet Archive, but they do have an American slant to what they attempt to give back. That is, if it isn't PD in America, then it might not be the best fit for them.

[0] https://archive.org/

[1] https://torrentfreak.com/