Saw this on the CS50 (Harvard's introductory Computer Science Course) Facebook group. Shared by the lecturer in charge.
Basically you have to have a (free) Edx account and I suspect be signed up (for free) to the CS50 course on edx (well worth doing).
Don't use Dropbox myself so can't verify if it works or how long it takes however commenters have mentioned that it seems to be working.
Please don't abuse this by mass creating accounts.