Can I add dropbox links to my resume for graduate school applications?
Your CV should stand on its own.  Admissions committees are likely to have stacks of material through which to look, and will almost certainly make a decision based on what you send them.  It is most unlikely that they'll be willing to hunt around to find things you should have told them directly, and even less likely if they have to download documents from Dropbox instead of just displaying them.

Spend your time making your CV as good as you can instead of spending time trying to take shortcuts, because that's what those Dropbox links will look like to the committee.