It's easy and helpful to use Dropbox on your Nexus 7 tablet to save files and apps. For blocking the unauthorized use with Dropbox files, you'd better set password to protect them.
How to Password Protect Dropbox on Nexus 7 tablet
* Open the Dropbox application, tap the menu button 》 Settings.
* Check the box with Passcode Lock under Dropbox account options.
* In setup screen, Tap "Turn Passcode" On, enter four numbers used as your passcode, and then confirm. If you want to delete them from the tablet after 10 failed passcode attempts, check "Erase Data".