Nexus 7 have only 8GB and 16GB storage, storing couple of things would run up memory and get low storage warning. Here is the tips on how to fix the issues.
* Delete downloaded files or stuff
Tap on Apps and open Downloads App. It contains all downloaded files, such as videos or music. Choose unwanted files and tap on "Trash" icon to remove them.
* Delete offline movies or tv shows from Google Play
If you have purchase and downloaded free/paid movies from Play store, they won't show up in downloads folder.
Tap on Google Play app, go to Movies > My Movies. There is a blue pin next with your offline videos. Tap on blue pin to delete movies.Then go to TV Shows > My Shows, tap on the pin to remove unwanted offline TV shows.
Notes: You can still play them on your tablet over WiFi network.