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Plex is a great client–server media player platform, that manage your videos, music, photos, etc… but Plex does not manage your ebooks (PDF, ePub, cbr, cbz, etc…).
And this is a major weakness of this platform, that Plex is not willing to address soon.
Manage both media (movie, music…) and ebooks (PDF, ePub…)
Calibre, in addition to Plex
Calibre is your first option.
Calibre is a cross-platform free and open-source suite of e-book software. Calibre supports organizing existing e-books into virtual libraries, displaying, editing, creating and converting e-books, as well as syncing e-books with a variety of e-readers.
But it required to two content managers. And the interface looks a bit old fashioned…
Jellybin in replacement of Plex
Your second option if Jellyfin.
Jellyfin is a suite of multimedia applications designed to organise, manage, and share digital media files to networked devices. Like Plex, il runs on multiple platforms: Microsoft Windows, macOS, Linux or in a Docker container (for your NAS). It is 100% free and open source.
The strength of Jellyfin is that you don’t need it in addition of Plex, it can just replace Plex. And it can handle both video, music… and ebooks.