As far as I can see, this is not, and never will be, possible from iOS because of the way Apple insist it must work. If you create a file in, say, Keynote, when you try to save it to iCloud Drive by "move to" you can move it to the PDFs folder, but then you cant open it in any way (And the original stays in the Keynote folder) In other words, Apple insists on forcing a folder-filetype-app relationship in iCloud Drive.
If you try to share it by saving it as a PDF the only option you have is "open in" - and since there is no App for iCloud Drive for you to open in, you cannot save it to a folder called, say, PDFs. Similarly, because there is no iCloud Drive app, you cannot look at your files in a folder and then use "open in" to get the right app to open them.
From iOS you can only save a file you have created in iCloud Drive if the app "knows" about iCloud DrivE, in which case, it does so automatically.
THis his is another example of Apples poor design and treating everyone like an idiot.
Bottom line, iCloud Drive doesn't help manage files, can't act as a way of saving device storage by use of the cloud and so falls far behind almost every competing cloud storage service. And where do I read the critical comment from techno journalists??? Oops, they all seem to have missed this.
Ps - and to add insult to injury, Safari on iOS won't let you open the web interface to iCloud to let you manage what is therein that way.