pf2e.fyi
From Pathfinder Core Rulebook, which isn't enabled.

Alchemist Armor Mastery (Level 19)

CLASS FEATURE 19
ALCHEMIST

Your skill with light and medium armor improves, increasing your ability to dodge blows. Your proficiency ranks for medium armor, light armor and unarmored defense increase to master.

Pathfinder Core RulebookOGL