You no longer need an always-on PC for Plex, if you are willing to pay more
Plex is a great software allowing you to access to your data from other devices as long as the computer, where your files are located in, is turned on. This means you have to keep your device turned on for a long time or you can use Networked Attached Storage (NAS), but in any case, Plex required an always-on device. However, you no longer need that, if you subscribe to Plex Cloud which keeps your files on Amazon servers. This requires a subscription to Amazon Drive and Plex Pass though.
In reality, Plex Cloud provides the same features as the normal Plex meaning opening your library to broad range of devices although Camera Upload and Offline Sync are not available yet. It is also worth mentioning that Plex Cloud does not have a support for DLNA yet, but I think that those missing features will come soon.
Also, note that, as there is no media migration tool, Plex Cloud is not a copy of your server.