I have a Mac, so I created a bunch of writable sparsebundle disk images ranging in size from 10 MB (single-PDF tax returns) to 1 GB (car documentation). I save them all directly to iCloud Drive. When I mount the disk image and make changes, only the changed bands are uploaded, avoiding a bottleneck or incomplete sync situation.
Time Machine keeps versioned backups of the iCloud Drive files on my offline backup disk (as of OS X 10.10.2). Periodically, I copy the disk images into OneDrive and Dropbox for redundancy.
Each disk image has a different password, all of which are secure (long strings of random characters) and managed well (saved on my Mac, as well as printed out and safely hidden in case of total disaster). My cloud accounts all have secure passwords, two-factor authentication, and all my computers have encrypted drives so I'm not out of luck if my computer gets lost or stolen.
For disk images that I know will never be modified again (e.g., Taxes 2003), I convert the disk images to a read-only format to save space in my clouds. I haven't paid a dime for cloud space, ever.
It sounds overkill when I type out the procedure, but because I've used only features built into the operating system, I can scan and archive a document in under a minute. On the other hand, this is complete vendor lock-in, so if I switch my primary computers from OS X down the road, I would have to throw out this entire solution and start a useless Ask Slashdot thread like this.
Hope this helps out any Apple nerds.