For apps to run, they need to be on the iPad's physical storage. Nowhere else.
You can delete Apps, and they will automatically become available from the cloud (i.e Apple's App Store Servers) for download as long as they continue to be available in the store. But you cannot move them to your iCloud purchased space,
In other words, you cannot select iCloud as a location for Apps.
Movies, Music, Books etc.. purchased from iCloud also work the same way. You can delete them to make space and will be available for download from the cloud / stores for free. But you cannot move them to iCloud.
For other documents and data like PDFs in certain Apps, and other files, you can move files, onto iCloud Drive from a computer. But to do so form an iPad would require an App that can access iCloud Drive. The procedure to upload would depend on the App.
For Photos, there's really no easy way to do this, on an iPad. As iCloud Photo Library, will continue to use local space on the iPad for the Photos it uploads, as well as Photo Stream.
Your best bet, is to determine what you do not need, and remove it, and what you need but less often, maybe move onto a computer. From there you can decide whether you want to upload some files to iCloud, and then be able to use Apps like Documents 5 to view them, and edit them.
https://itunes.apple.com/en/app/documents-5-fast-pdf-reader/id364901807?mt=8