When you reveal a unit with Paaka Protector, does it have to go to hand or can you trash it for +2?

No — if the revealed card is a unit, you must draw it. The +2 Shield only happens when the revealed card is not a unit.

Pakaa Protector's ability is a strict if/otherwise effect with no "may": "If it's a unit, draw it. Otherwise, put it in your trash and give me +2 [S] this turn." You can't choose to send a revealed unit to the trash instead of drawing it, so there's no way to trade the unit for +2 Shield.