As usual, the answer is "it depends".

If you are only using the Cloud for storage, it's relatively easy; you can simply encrypt the document before uploading, and then decrypt when you need to download it. 

However, if you need to be able to search, index and manipulate documents in the cloud, you currently basically need to trust your cloud provider. There have been proposals using homomorphic encryption, but as far as I know they are not ready for prime time.

I have worked on a number of "Ivory tower" solutions that try to address the second alternative to varying degrees - see enclosed.

Article The design of a redundant array of independent net-storages ...

Conference Paper I’ll Trust You – for Now

Conference Paper Securing big data in the Cloud by protected mapping over mul...