Given their integration into the Mac’s Finder, it can be easy to forget that online file sharing services like Dropbox, Google Drive and Microsoft OneDrive can be accessed using a Web browser by anyone with your username and password. iCloud Drive is a bit more secure because you have Two-Factor Authentication turned on as we have recommended! Obviously, you should always have strong, unique passwords, but to be safe, it’s best not to use services designed for public file sharing to store unencrypted files containing sensitive information like credit card numbers, Social Security numbers, passport scans, privileged legal documents, financial data, and so on. Keep such data secure on your Mac—outside of any synced folders—where accessing it requires physical access to the machine.
For sensitive documents and information we always recommend storing them in 1Password. That way they are secure and available on all your devices.