I can tell you what we ended up doing. We have a download (handout) page near the beginning that has a link button which we have going to the attachment we uploaded to our server. We also have that slide set to advance by user so they will need to actually press play in order to move forward.
Is there a way to ensure the link always opens the attachment in a new tab/window?
Test it, which I would think you should always do. Not sure how it would handle it in a browser set as the default program for opening pdf's. Let alone the human factor of people not reading and paying attention.
Sorry for the delay in response.
As suggested by Melanie, for HTML5 output attachments are not available by default.
However, when attachment files hosted on a server are linked to the presentation hyperlinks, they do open in a new tab.
I gave a try for Chrome, Firefox and IE. Let me know if you face any issue.
Adobe Presenter Engineering Team