To get Pages Documents (and Word) to iCloud from your computer, log into iCloud.com and go to the iWork tab. Then copy that file to the page. Works with Numbers and, I think, Excel files too. But you can't copy any type of file you want to iCloud, it has to be one of the supported formats. Also, you need to have Pages/Numbers app on a device to allow syncing from iCloud to the device and to copy from Mac to iCloud.