I'm scratching my head on this one too. Apple says "Preview in Mountain Lion supports Documents in the Cloud so you can access your PDF documents and images from anywhere." They use the "anywhere" terminology other places to mean even from your iPhone/iPad/iPod. I'm having a hard time believeing they made it possible, but didn't give us a tool (app) to actually allow us to do it.
It is a little mystifying. I think the reasoning is that most people don't understand that files can be opened by multiple applications - for them, they simply live inside Word, Excel, or whatever (seriously - everyone I work with thinks this way and doesn't know how to open documents except through an application).
So Apple is making this the reality and all documents can only be opened from the application that created them. I'm not sure I get the usefulness of this, especially since many OSX applications have no iOS counterpart, and access to documents on iOS devices is what most people use iCloud for. At the very least, the current implementation is pretty useless when Preview can "own" any file.