It’s secure! I use it myself. Dropbox stores users data at Amazon’s S3 cloud.
Dropbox takes the security and the safety of your data very seriously. They use the best tools and engineering practices available to build and maintain Dropbox, and have a dedicated security team making sure that Dropbox remains secure. Our files are stored securely and backed-up. Your account login is protected by many layers of security including password and two-step verification.
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Read more here: https://www.dropbox.com/help/27/en