I'm not recommending any particular cloud storage service. Instead the topic is how to keep your documents secret!
I am big fan of Google Drive. But regardless of which service you use, I see no reason to trust that the company providing the service is not rifling through the information you have entrusted to them. Well, I guess I'm screwed then, right? But this is where Cryptomator (https://cryptomator.org/) does it all. It is a cross-platform tool that creates an encrypted vault where you store those files that you want to be sure are only accessible by you.
There is another similar solution that I have not tried called Boxcryptor (https://www.boxcryptor.com/).