There's a minimum 1 point of non lethal for attacks usually, eg an 7 str character rolling 1d8-2 damage who rolls a 1 doesn't do -1 damage (healing the enemy), the minimum is 1 non lethal.
However that doesn't gappen when it's DR reducing the damage. The minimum for DR is zero damage.
It's somewhere in the combats chapter or chapter 1 in the CRB. We stumbled upon it a few weeks ago.
It's here:
If your attack succeeds, you deal damage. The type of weapon used determines the amount of damage you deal. Damage reduces a target’s current hit points.
Minimum Damage: If penalties reduce the damage result to less than 1, a hit still deals 1 point of nonlethal damage (see Nonlethal Damage).
But also ...
Some magic creatures have the supernatural ability to instantly heal damage from weapons or ignore blows altogether as though they were invulnerable.
The numerical part of a creature’s damage reduction (or DR) is the amount of damage the creature ignores from normal attacks. Usually, a certain type of weapon can overcome this reduction (see Overcoming DR). This information is separated from the damage reduction number by a slash. For example, DR 5/magic means that a creature takes 5 less points of damage from all weapons that are not magic. If a dash follows the slash, then the damage reduction is effective against any attack that does not ignore damage reduction.
Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury poison, a monk’s stunning, and injury-based disease. Damage reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact.
Attacks that deal no damage because of the target’s damage reduction do not disrupt spells.
Yeah, you're damage calculation can never result in dealing less than 1 non-lethal damage.
But damage reduction can certainly reduce that to 0, in fact that's the desired state (to reduce damage to 0).