DocSend users have the ability to allow or disallow the downloading of their documents from the document viewer. This is controlled on a link-by-link basis and can be changed even after a link has been created and shared.
If you are viewing a document, and downloading has been enabled, you will see a "download" icon in the upper-right of the document viewer:
If downloading is not enabled for the link you're viewing, the "download" icon will be missing. The owner and sender of the link you're viewing can easily allow downloading by following the steps listed in this article: How do I enable downloading for a link I'm sending out?