I have encountered my first “Documents and Data” issue taken all the available space on a iPad( iPhone has the same issue) If you only have 16GB version then the chances of this one appearing are high.
After loads of searching with re-directs to 3rd party software sites to sort this issue out or even worse case a backup factory erase and reinstall the solution is actually a lot easier than the search engines would like to tell you. Purchase they get royalties from you buying the 3rd party app which will sort out all your problems or just that the internet is limited about this issue with the OS device. I don’t know if any of that is true but the fix is very easy.
Firstly my assumption is you are running OS 9 and that you have checked your iTunes, video and photos apps. I say this as I have had a few iPads come in and it was one of those that was causing the iPad to run out of room. Good then go to settings, General, Storage & iCloud Usage, Manage Storage, then wait and I mean wait for all the apps to populate. The spinning wheel will stop once it populated all the apps, look at the list and identify the culprit app causing the issue. Delete the app in question, no you cry well if you want the storage message to stop popping up then you will have to delete the app in question. Once done simple go to the app store, purchases and download the app again. Once installed open the app and then with the app open go back to Manage Storage look at the apps and see if the culprit app has been significantly reduced in size. If not then consider not having that app on your device as it will always cause the documents and data issue because the device is not big enough to hold all the apps.
That is it simple really no need to buy any addition 3rd party software to “clear out your device of unwanted files”.
Settings, General, Storage & iCloud Usage, Manage Storage check the apps size, delete the app is question, reinstall it if you want, open the fresh download of the app check back at manage storage to see if its gone to a sensible size and off you go.
If this has not worked then I would go for the backup in iTunes, restore to factory settings re install from backup, but this is a last resort.
hope that helps.