A few days ago on the 5th of May, Dropbox has disabled shared documents that was shared before the said date. Apparently Dropbox did it to prevent files shared by users to be opened by unintended parties.
Previously anyone who has the link to a shared dropbox file was able to access the file freely without any security measures in place whatsoever, intended or not. Dropbox at the time failed to realise that this was quite a massive security flaw. Ironically Dropbox did not stumble upon this security flaw through any reported incident (because fortunately there were none) but through a comment in its competitor, IntraLinks’ blog mentioning about the potential security breach that can happen.
Therefore, upon realising Dropbox’s accidental security leak it decides to disable all the older shared links with no security measure. To share a document in dropbox now requires users to turn on a setting which restricts the viewership of the shared documents to intended members only. To create a new shared link follow the instructions here.