Everyone's needs are different but I don't want my files spread across multiple app containers. I like having all relevant files in one folder, just like you would organize your files in Finder on your mac. A cloud drive app that would allow viewing of PDF/keynote/pages is perfectly within the capacity of Apple's programmers. I can understand having the specific apps launch when editing is necessary (like google drive/docs/slides).
If other people didn't like organizing their files this way, google drive and dropbox wouldn't be popular apps. These are good solutions to file management on a mobile device. I was hoping to move my files to iCloud to keep everything "apple," but if not that's ok because there are already other services that work well.
There is no built-in PDF solution, obviously. There is an app for everything, but this discussion board is for apple products/services.