First time you want to attach a file to your event you will be asked to give app access to your Dropbox.
By granting access only the app is able to see your Dropbox files, you are not giving away your login and password.
Attaching a File
When you attach a file to your event Dropbox will make this file accessible.
A unique URL, which looks like this
<a href="https://www .dropbox.com/s/i3bnzp026xd73vl/example.jpg?dl=1">example.jpg</a>
will be generated and is placed in the Notes section of your event.
Only people who know this unique URL will be able to see the attachment.