It doesn't have to be, according to the current Dropbox Terms of Service[1]:
>We need your permission to do things like hosting Your Stuff, backing it up, and sharing it when you ask us to. Our Services also provide you with features like photo thumbnails, document previews, commenting, easy sorting, editing, sharing, and searching. These and other features may require our systems to access, store, and scan Your Stuff. You give us permission to do those things, and this permission extends to our affiliates and trusted third parties we work with.
This is unarguably something that facilitates searching.
>We need your permission to do things like hosting Your Stuff, backing it up, and sharing it when you ask us to. Our Services also provide you with features like photo thumbnails, document previews, commenting, easy sorting, editing, sharing, and searching. These and other features may require our systems to access, store, and scan Your Stuff. You give us permission to do those things, and this permission extends to our affiliates and trusted third parties we work with.
This is unarguably something that facilitates searching.
[1] https://www.dropbox.com/terms