Safe 3rd Party File Storage, easily integrated with WP

I’m building a membership site, which will make some reports with fairly sensitive information available for downloads. For safety reasons, I’d like to keep these files hosted with some service specializing in safe upload/download. Looking for suggestions.

Though client area will be password protected, occasionally, wp accounts get hacked even on best hostings with 0 plugins on the site. So, there is gonna have to be an extra layer of protection offered by this service, easily integrated with my site.

The plan is to have up to 10000 users on this site. So, I’m also going to need a way to assign my uploads to each of them. Each of my site’s accounts will need individual access protected.

Has anyone heard of any services like this?

Thanks in advance.