I stumbled on this old FAQ entry which contains this sentence:
If it's a confirmed critical hit on a sneak attack while employing Power Attack with a flaming greataxe, the single damage roll is 3d12+3xStrength+2xPower Attack+1d6 sneak attack+1d6 fire.
I know the entry is about Society but I guess the way to calculate damage should be the same as in standard Pathfinder. I always thought the Power Attack bonuses were multiplied by the critical factor of the weapon, not only by two. The description of Power Attack states nothing special so I don't think I am wrong, but I would like to have a confirmation.
Best Answer
That is a typo
There are examples on published adventures of how power attack works when being multiplied. And an official ruling has been given by Jason Bulmahn (Lead Developer):
And again on this thread, when answering a question about Mythic Power Attack.
When bonus damage shouldn't be multiplied on a critical hit, it is mentioned in the effect (ex: Raging Brutality and Vital Strike). Otherwise, it's noted as Precision Damage, which never multiplies (ex: Studied Combatant)
We have this on the combat chapter as well: