add download attribute to download link in gray header

[sfsheath]

We forgot that. Also, looking at https://www.w3schools.com/tags/att_a_download.asp I see that filename can be set. Does this work in current browsers? If yes, we can get rid of duplicate file, which would be nice.

[sfsheath]

I now see the download attribute, but on Safari and Chrome, clicking on the link loads page instead of downloading. Also file name in attribute should include -offprint . I tried a few papers, including http://isawnyu.github.io/isaw-papers-awdl/13/ .