i think what you are searching for is a media server theire are planty of them out their. An open source variant would be jellyfin. Its not directly integrated in nextcloud but i think with a bit tricking around you should be able to hang in a nc share and use jellyfin as browsing library (works and looks simular to netflix). Its possible to store music, films, series, books and stuff.
I have done exactly this since I run them both in Docker on the same server. I have a group folder with media, and then mount that folder from within the data folder as read only in Jellyfin. Works like a champ.
If it’s just PDFs, seems like Nextcloud could serve them up just fine.
Looks like a very advanced variant of what I am looking for. But it doesn’t really fit. I would appreciate all the fun stuff around like movie server and such. But the server I am running my documents on is not located at my primary home. The reason is simple, on my second location I have plenty of solar power and so the server is running “for free”.
So I am really looking for a “document server”. I would like to have an overview page of what I have but also would like to have the PDFs searchable or even auto-indexed?