There is a difference between sharing a Dropbox folder and a Dropbox link. The former is not always accessible (for some reason) to the general public whereas content shared via a Dropbox link appears to be nearly universally accessible.
Case in point, I shared a Dropbox folder via email to All Glass Parts in Canada and Alan was unable to open it. I sent the same content via a Dropbox link and the material proved accessible. By the way, All Glass Parts has over 6500 items in stock. They couldn’t help me but were very nice in terms of suggested alternatives.