The downloading of files cannot be restricted once they're published on a cloud space, you're better off not uploading anything to the Internet to begin with, and opt to not use any commercial software at all (but this is just my opinion).
However, you may encrypt files with password to restrict the opening and the viewing that was meant for a specific user, and you may also control the sharing of such files.
If these files have such sensitive information, and they will be shared over a cloud space somewhere on the Internet, I'd suggest some careful thinking about the networking workflow (routers, VPNs, blah, blah, blah), and the possible impacts in the event of a breach.
Also, you may want to consider how the users will gain elevated privileges to some network locations before they hit the intended cloud space.
But then again, you can't control what the users could do with such information once they've gained privilege to opening such files, amd how will that document is shared further via email, printed as a hard copy, scanned and faxed.
You may not even be able to prevent information leaks that also seems very common in today's work environment, where irresposnsible users also seem to be very creative with their mobile devices, like recording a video or taking a picture of the computer screen with their cell phones... Sadly, we can't prevent these things.
But, The link below addresses how to prevent certain actions when a shared PDF is forwarded via email, for example :
In addition, you may even want to implement an expiration time to lock or restrict continued access for such documents from the moment a user opens them.
There may be third-party services that can provide that, and even offer you other solutions, like hiding IP addressing, encrypting, and sending and tracking such documents anonymously, but it may cost you money.
In the end nothing that we do online is really that private or confidential anymore. You may make it difficult for others to have access to, but that's about it.