OH MY GOSH! I'm surprised there are not more people complaining about this. 
So far, I have only discovered a few other threads regarding this issue.

This QUICKBOOKS ONLINE issue..IS SO FRUSTRATING

(is this also happening in the desktop version??)

QUICKBOOKS ENGINEERS

What setting did you turn on/off that is preventing users from being able to view their PDF file attachments in a new tab or popup window????

I do not want to change all my computer doc defaults to a different file type from PDF just because QBO isn't supporting it anymore.