Q: How do I backup Finder documents to iCloud?

If you have upgraded your Mac to Yosemite and your mobile devices to iOS 8, you can upgrade  iCloud to iCloud Drive. Then you can store documents in iCloud by copying them to the iCloud Drive icon in Finder.  See: iCloud Drive FAQ

How do I backup my Finder documents

But note, copies of your documents on iCloud Drive are no archival backups. iCloud Drive is primarily meant to keep your documents synced across all devices. If you change a document on any of your devices, the change will sync to all devices and you cannot restore the previous versions. To backup a Mac it is much more convenient to backup to an external drive with backup software like Time Machine. That has the added advantage, that you will always be able to access your backup, even if the network should not work, and it is faster and cheaper than paying monthly fees for cloud storage.