Is an employer PPO health plan the same as a HDHP?

No.  A Preferred Provider Organization (PPO) is a type of traditional health plan that contracts with medical providers, such as hospitals and doctors, to create a network of participating providers.  These plans usually have lower deductibles but higher premiums.  

A high-deductible health plan (HDHP) is a health insurance plan with lower premiums and higher deductibles than a traditional health plan.  You must be enrolled in one of these plans to have an HSA, health savings account.  

If you are unsure, contact your HR department and they can clarify what kind of plan you are enrolled in.