Are you 100% sure it's even partical board and not OSB? Totaly differnt materails.
Partical board looks like compressed saw dust and is no longer an exceptable subfloor. If it gets wet once or there's rising moisture from under the subfloor it turns to oatmeal and is useless.
If it is partical board cut it all out and replace it with 3/4" Advantec T & G subflooring glued and screwed to the floor joist for a subfloor that will be level, far more moisture resistant and will be flat.