Upon reaching 9th level RANGER class, the ranger gains the evasion special ability.
Current text for evasion says the following:
You can avoid even magical and ususual attacks with great agility. A successful Reflex saving throw against an attack that normally deals half damage on a successful save, you instead takes no damage. Must by in light armor or no armor and not helpless (PFCR 68).
FOR RANGER, he can also wear MEDIUM armor (PFCR 67), not just light or no armor:
Evasion can be used only if the ranger is wearing light armor, medium armor,
or no armor.