iCloud storage in Finder
There is a folder, located at ~/Library/Mobile Documents/, that iCloud-enabled apps store their data in. You can trickily use this folder by simply putting any files in there that you want to access from another iCloud-enabled computer. Unfortunately, there's no way to access this folder via the web or on an iOS device, but Macs synced to the same iCloud account will all have your files in that location. I'd recommend creating an alias of some sort as well, to get quick access to it.

In answer to your P.S., there is no way to access this folder on Windows, as it doesn't exist. If you are for some reason only wanting to move iWork files, you can use and access the iCloud web interface on any computer, but I doubt this will help you.