Dropbox controls what users upload on the platform and they remove, let's say, unlawful content. That's in their ToS!
How can they check for such a content if they can't see (or don't see) what's there?! 🤔🙂 Impossible, of course!
If you want to keep something secure, take care. Don't rely on wishes. 😉 There are lots of encryption software. Encrypt your "database" before upload/sync to Dropbox (or any other cloud service) and you will need to know only one password key. Choose a strong encryption!