The Hoarder is one of the achievements and trophies available in Dead Island: Riptide. To unlock this, you must find all the pages of the Palanai Island Guide, all of Angela Guerra's Recordings, all the Postcards, all of John Morgan's Diaries and all the Secret Files. They can be found and picked up in any order, but they must all be found for this to unlock. If you need help with where they are, see their dedicated pages for locations of them all. Some are tricky to find and get, but once you have all of them, this will unlock.
Dead Island Wiki