It used to be the case that the only files you could store on iCloud were from specially-built apps such as Apple's iWork suite, but now we have the upgraded iCloud Drive in macOS Sierra.
Now, in macOS High Sierra, you can chuck any file you like onto either the Desktop or Documents folder, in addition to the iCloud Drive icon in the Finder sidebar, and those files and folders will be synced automatically.
Now, in macOS High Sierra, you can chuck any file you like onto either the Desktop or Documents folder, in addition to the iCloud Drive icon in the Finder sidebar, and those files and folders will be synced automatically.
Those special, “blessed” apps still get their own folders, but you can create your own or just put things into the iCloud Drive loose via the Desktop and Documents folders.
All those files will sync to other Macs signed in with your Apple ID (so long as you've enabled iCloud Drive on them) and will also be available through icloud.com.
All those files will sync to other Macs signed in with your Apple ID (so long as you've enabled iCloud Drive on them) and will also be available through icloud.com.
On iOS, apps that can use iCloud will usually default to opening files from their special folder, but should also allow you to browse through your entire iCloud Drive to open files stored elsewhere.