It's a loooong journey before I found the answer. Tons of suggestions on the Internet and some one even came up with a very good long check sheet.
However, my problem is probably simpler but hard to find the answer. It caused by the printer port.
They said Windows 7 64-bit would always point to LPT1 port as default. QuickBook needs to use NUL port instead of LPT1. So it's simple, all you need just change this setting from QuickBooks PDF Converter -> right click -> Printing properties -> Ports -> check on NUL -> Apply -> Restart computer.
So if your Windows is 64-bit, chances are you are having the same problem that I had before. It took me 2 whole days to debug this. Lesson Learned!