Doubling and Halving Damage
Source Player Core pg. 399
Sometimes you'll need to halve or double an amount of damage, such as when the outcome of your Strike is a critical hit or when you succeed at a basic Reflex save against a spell. When this happens, you roll the damage normally, adding all the normal modifiers, bonuses, and penalties. Then you double or halve the amount as appropriate. As normal, round down if you halve the damage (though 1 damage halved remains at a minimum of 1 damage).
When doubling, the GM might allow you to roll the dice twice and double the modifiers, bonuses, and penalties instead of doubling the entire result, but this usually works best for single-target attacks or spells at low levels when you have a small number of damage dice to roll. Benefits you gain specifically from a critical hit, like the extra damage die from the fatal weapon trait, aren't doubled.
When doubling, the GM might allow you to roll the dice twice and double the modifiers, bonuses, and penalties instead of doubling the entire result, but this usually works best for single-target attacks or spells at low levels when you have a small number of damage dice to roll. Benefits you gain specifically from a critical hit, like the extra damage die from the fatal weapon trait, aren't doubled.