You can enter a bogus email address and still be able to download the PDF. They're more concerned with you not being a spambot, so as long as you pass captcha, it doesn't matter what email address you use.
If you do register, you can uncheck all the notification boxes under preferences. Bottom line is you can have as much or as little interaction with issuu.com as you want. That's one of the reasons the eZine is posted there, in addition to allowing people who are not aware of its existence to discover it.