@rjmelville 

I also had this issue back in November.  Everything was working fine after the December update and into January until today 1/11.  I started experiencing the same issue and checked with my other offices running separate QBO companies and they are having the same problem again.  Every attached pdf through yesterday can be viewed as a separate browser window, but today it's bringing up a "Save As" window to put in the Download file.

Are you having the same issue again?