A possible temporary work-around for you Mac / Intuit Payroll users: instead of "launching" Payroll from within the QB-for-Mac application, go directly to your browser (Safari/Firefox/etc) and log in to Payroll there (at iop.intuit.com).  You should now find that your browser launches whichever PDF viewing app you use, per its normal behavior.  Thus the problem does seem to be with Quickbooks' implementation of Safari-within-the-app.  Wouldn't it be nice if they'd give us a Preference setting where we could choose our PDF viewer?