A 87 Damage Reduction is an ability some creatures possess. The section on firearm penetration range says firearms are not considered touch attacks for ability interaction purposes, just that they check against touch AC within certain ranges. Thus, firearms must contend with damage reduction like any other weapon.
This doesn't seem to be the first time this pair of rules has come up and there doesn't appear to be a FAQ or other official ruling that can be easily found to clarify, so you may have table variance on this. The intent of the note for damage reduction (which that sentence only appears in the Core Rulebook and not the Bestiary entry for Damage Reduction) is so casters, like clerics, don't have to punch through DR to deliver spells, harmful or otherwise, to such creatures, as otherwise you'd have situations where Barbarians of 7th level or higher were immune to Cure Light Wounds and its bigger brothers for the fact of having DR 1/-.