download triggers popup blocker; users aren't seeing it and think it's broken

On this page

http://joeldcanfield.com/download/the-inconceivable-adventure-of-jake-calcutta-and-the-primary-source/

when I submit the correct password, the tiny “popup blocked” icon shows up and I can’t download until I allow popups. I don’t want every user who download the file to reconfigure their browser. Is there something I’m missing?