Currently, only Apple apps (Pages, Numbers, etc.) can back up
data to iCloud. There is no way to use it to just simply backup any files.
There are some workarounds to get some data stored.
PDFs and any other document that can be read by Preview
can be backed up in iCloud. Load the document into Preview,
select Export..., click on the directory dropdown, then select
iCloud as destination. Not a drag and drop solution or an
all encompassing method, but you can get some stuff saved.
Generally no, icloud is not a real storage service, it's primarily for syncing between devices and performing backups from iOS devices to icloud. As mentioned above, only those apps that have use of icloud programmed into them can use icloud to store data or sync to other devices. You need to look at Dropbox or Sugarsync.