We need to restrict some documents from being downloaded locally in certain situations. Our teams share some docs that are perfectly fine to be shared throughout their teams while others are more sensitive and require a more limited circle of sharing.
We are hacking it by using (and paying for) DocSend (by Dropbox) and sharing the link via HubSpot to maintain some level or trackability, but would save us time, effort, and money to have this feature added to Documents.