Hi, I totally agree that Apple should offer an option to delete the local copy of files downloaded from iCloud Drive to an iOS device (like Dropbox does). I could not find a way to do so after googling and looking all over the different option in my iPad. Nonetheless I found a workaround to recover the space used by videos and pdf files, etc downloaded from iClould drive to my iPad. Unfortunately this workaround removes everything that has been downloaded altogether, but at least I was able to recover the space. If in the future if for some reason I need a local copy of a file I had previously downloaded , I will have to download it again, which in my case does not seem to be a great deal.
I am using iOS 10 and I do not know if this works in other versions.
Here is what I did :
1) This step is to check that this procedure works for you. So, in the iCloud Drive app, look for a video or pdf file that when opening it does not ask you to download it. That is a file that has already been downloaded and resides as a local copy on your iOS devices. Take note of the location and name of that file. Also take note of the free space available on your iOS device, under Settings > General >Storage & iCloud Usage > STORAGE section > Available.
1) Disable iCloud drive by going to Settings > iCloud > iCloud Drive > Turn iCloud Drive Switch to OFF.
2) Wait some seconds and Enable iCloud drive again by turning the iCloud Drive Switch to ON.
3) As an extra measure, restart your iOS device.
4) After restart completes launch the iCloud Drive app and be patient, it will take several minutes to reload all your folders in iCloud Drive. In my case I am interested in the Documents and Desktop folders from my Mac running MacOS Sierra.
5) Once you see your folders as you would on you Mac or PC, go and look for the file you tried on step 1), you should now be asked to download it, which indicates that the file is no longer in your iOS device. Also, the available storage in your device should increase noticeably if you had downloaded a significant amount of files and/or large files previously.
I hope this helps, although again, this is by no means the way things should work in the first place, but for me it was better that being stuck with a bunch of files I did not need on my iPad anymore.
Regards,