You already called out the problem: you can only be up as often as all of your critical partners are up.

If you care about uptime and reliability, you can only partner with well established, reliable players for core functionality. For example, Google Maps is a pretty safe partner.

Also, what would you do if they went away? I'd make sure you have your own copies of every document, just in case.

Now, in all fairness, scribd has been up whenever I've used it and seems to be continuing to gain traction.