It seems ticket #15899 had similar issues with the pdf.js download button, I don't see any errors in the console. It may be worth re-opening this ticket. I see similarly no activity as a result on clicking the download button on pdf.js running on Tails 2.4.
As a work around Right Click
-> Save Page As
(do macs even do right clicks yet?) and File
-> Save Page As
should both still work.
Update: This bug has been fixed under #19401 a patch is now applied in Tor Browser 6.0.2