This app was mentioned in 4 comments, with an average of 1.75 upvotes
Well let's say you have some photos that you want to hide... like maybe work-related snapshots of the whiteboard after a meeting.
Use an app that can hide photos: https://play.google.com/store/apps/details?id=com.colure.app.privacygallery
Also, the Dropbox app can be secured behind a PIN, so if you use their cloud storage service to backup files then you can also make that app harder to get into.
Hide Something is very good at hiding photos and videos. Not sure about documents though, I personally don't use one to do that.