If a link you've shared has stopped working for someone, check the following:
You should only use shareable links for files that you have the legal right to share with others (for example, documents, photos, software, music, and videos that you personally created). You do not have the right to share files unless you own the copyright in them or have been given permission by the copyright owner to share them. Purchasing or legally acquiring video, music, ebooks, or software does not give you the right to share that material with third parties over the Internet.
If you have any doubts about whether you have the legal right to share certain files, you should not share them. You represent that files shared using shareable links do not violate Dropbox’s Terms of Service and that you own all copyrights to them or have been given permission by the copyright owner to share them. Dropbox has adopted a policy of terminating the accounts of users who repeatedly infringe copyright or whose accounts are subject to multiple infringement allegations. If you repeatedly share files that infringe others’ copyrights, your account will be terminated.
For more information on Dropbox’s copyright policy, please visit Dropbox’s DMCA page.