You cannot tag on iOS, but can on OS X. That's what I mean by all the pieces are in place... They just haven't been put together in anything resembling a useful way.
Speaking of which, one of the big advantages of iCloud Drive was the ability to link apps together into workflows. When I opened Pages in Dropbox, all the files were in the .pages format, which is supported by... Well not many apps.
Surely there is a way around this. The pages app itself can export as pdf or doc, but iCloud doesn't appear to be able to access this.