Do not touch the PlexMediaServer shared folder. It is where Plex Media Server stores its database, log files, etc.
The PlexMediaServer user needs at least Read Only access to the folders where your media is located. It will need Read/Write if you plan to use Optimized Versions, LiveTV/DVR, or other features where Plex writes to the media directories. See the DSM 7 - Set permissions for (Media) Shared Folders section of DSM 7 - Plex Media Server - Release Info & Migration instructions for additional details.
