I've a strange comportement in SP2013. When a user add a document in a library. He is the only person that can see documents added.
Do you know which parameter could do that?
Has the user checked in the documents? The documents he added might still be checked out to him.
Go to Library settings -> Manage files which have no checked in version to see these documents.
User must check in document (and publish) it so other users can see and edit it.