Q: Is it possible to back up Word and Excel documents from my Mac to iCloud?

No. The only devices that can backup to iCloud are iPhones, iPods and iPads. Mac cannot.

Well, I just tried an experiment.  I loaded a ".docx" document onto iCloud Drive from my Mac.  Indeed I was able to locate and access the document from other devices.

So iCloud Drive may fulfill your needs.  You said "backed up to iCloud", and indeed no Mac files are backed up there per se.  You would have to consciously move the files there.

Here's more information:

OS X Yosemite: Store documents with iCloud Drive

sberman wrote:

 

Well, I just tried an experiment.  I loaded a ".docx" document onto iCloud Drive from my Mac.  Indeed I was able to locate and access the document from other devices.

 

 

That is not a backup, and it is foolish to use it as if it were. Backups are not synchronous.

Gail from Maine wrote this excellent comparison of Backup and Sync.

Re: What is the difference between Sync and Back Up?

Yes, as I said in my second paragraph, this is not a backup.  But it's tough to tell if backups are really the OP's objective

After all, Time Machine provides an excellent way to back up everything on a Mac, whether from a Microsoft product or anything else..

I saw, but "moving the files there" is also not a backup, in fact after moving them they are the only copies, copying them there would be a better idea, Time Machine and iTunes would be the best.

And if the OP really means sync instead of backup we can start again

I agree with much of what you just said.

Time Machine - sure.  Copying - sure, if backing up is the objective.  But I don't see how iTunes relates to this issue.

iTunes to back up data from the iPhone to the Mac, Time Machine to backup the Mac (and all iCloud data as well)

Right ... except the OP never mentioned an iPhone.  If one is involved, then sure ...

Not specifically for this post, a general rule if an IOS device is involved, and I recommend a completely separate backup of the Mac via a clone as well.