I am reporting back on this. After I checked in at the airport counter and received the boarding passes for both flights, I still could have CX.com to generate a PDF boarding pass. CX.com kept telling me that online boarding pass is not available for this flight due to either airport restrictions or connecting flights operated by partners. So it seems that as long as the onward flight is not operated by CX, it is not possible to generate a PDF boarding pass from CX.com (even for the ex-HKG flight)? Is this true? Or what I experienced is just an exception? Thanks!