DB syncs what ever the content of the file is, so if you can implement a file encryption system on the PC and phone that can encrypt and unencrypt the files then DB will sync that file in its encrypted state. But you need that encryption/decryption at your end(s).
DB does offer two-step authentication for your entire account.
Two-step verification adds an extra layer of protection to your account. Whenever you sign in to the Dropbox website or link a new device, you'll need to enter both your password and also a security code sent to your mobile phone.