What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I'm not going to say that dropbox integration is 100% secured, but from what I know they uses Secure Sockets Layer (SSL) and AES-256 bit encryption and uses modern method when tranferring and storing data.
Dropbox also do a regular test for security vulnerabilities to enhanced and protect against attacks. For added security they use the Time-Based One-Time Password for login. Also when accessing drop through integration they use authentication to connect and user will need to grant some access to use dropbox.
You can add extra security to your form by making it SSL. Please check thig guide on how to receive SSL submissions http://www.jotform.com/help/63-How-can-I-receive-SSL-Submissions.