Q: Using iCloud Drive on iPad as a backup for documents, music and photos

You can check out iCloud Drive FAQ, OS X Yosemite- What is iCloud Drive?, Get help using iCloud Drive - Apple Support, Set up iCloud Drive on all of your devices, and Get help using iCloud Drive. Of course you will need iCloud Drive for Windows.

One issue that occurs to me is that if you accidentally delete something on the iPad Pro it will also be automatically deleted from iCloud. There are ways to recover something on iCloud if you notice it within 30 days -- but if you didn't notice within that timeframe it would be gone. With a good backup routine from a computer to an external hard drive that would not be the case.

One option might be to use both OneDrive and iCloud. I have saved Microsoft Word and Excel documents to OneDrive from an iPad and I have saved all types of documents (i.e., documents from non-microsoft applications) to OneDrive from a computer. I have not tried saving non-Microsoft documents to OneDrive from an iPad but my understanding is that it is possible. The following app (which I also have not tried) may be of interest: https://itunes.apple.com/us/app/onedrive-cloud-storage-for/id477537958?mt=8

Note that to Microsoft Word and Excel are free on iPhones/iPads with the exception that to create or edit files from an iPad Pro you will need an Office 365 subscription. If you go that route, be aware that the subscription includes 1TB of storage on OneDrive.

Try backing up the phone to iTunes.