Plex has something called Plex Pass which gives you some extra features, like
cloud synchronisation of files, but this isn't always necessary.
The real gem for Kindle owners though is Whispersync, Amazon's integrated
Cloud synchronisation software which not only backs - up purchased content but also synchronises your furthest reading position across multiple Kindles or devices using Kindle apps.
Not exact matches
The optional additional
cloud - based services provide advanced bookshelves and annotation
synchronisation, and support flexible sharing models.