Any iOS app can let its users pick a file on iCloud Drive with the proper chooser dialogue. This dialogue allows to search files based on tags. Here is a screenshot to make crystal clear what I am discussing here:
So some files on my iCloud drive have tags, which I added using the Finder on my mac running Yosemite. Typing the tag in the search box above results in an empty list: no file is found.
This is in direct contrast with the Finder on MacOS where search tags does work as exemplified by the following two screenshots:
I would appreciate if somebody else could either confirm the problem, or on the contrary find that searching by tags works fine. I tested on an iPad Air running iOS 8.3 which is the most recent version at the time of writing.
Note that Keynote and Pages use this file chooser, to pick images e.g.