Mayweather would probably kick Pacman's ass. Pacman has always had trouble with guys that could box well such as Barrera, Marquez, and Morales. Right before moving up in weight he barely won against JMM.
All these fights did happen before Roach became his trainer though, so I'm not sure how he'd do against against a guy like Floyd.
Floyd is basically a much bigger version of those guys with a lot more power and much better boxing ability, dude is crazy good, he's bulked up a bit since moving to 147 and is still at the top of his game.
The ONLY punch I've ever seen that has bothered Mayweather was the jab, something Pacquiao doesn't really have. Zab Judah was snapping Mayweather's head back and so was DLH when they were throwing the jab at Mayweather. Judah of course started off his fight like he was P4P #1 but wound up choking in the later rounds just like he has his entire career, DLH was too old and slow for Mayweather and didn't stick with the jab for some reason.
Mayweather is crazy talented, his defense is amazing, he's also got a very solid chin. He took a monster shot against Moseley but was able to hang on and keep his composure.