A Study to Learn About the Study Medicine Called PF-07868489 in Healthy Adult People and in People With Pulmonary Arterial Hypertension

Key Inclusion Criteria Part A: * overtly healthy * Body mass index (BMI) of 16 to 32 kg/m2; and a total body weight \>50 kg. Key Exclusion Criteria Part A: * clinically significant hematological, renal, endocrine, pulmonary, gastrointestinal, cardiovascular, hepatic, psychiatric, neurological, infections or allergic disease. * smoking more than 10 cigarettes (or equivalent) per day or smoking history ≥10 pack-years. Key Inclusion Criteria Part B: * diagnosis of pulmonary arterial hypertension (PAH) * stable dose of standard of care PAH vasodilators * BMI 16 to 40 kg/m2; and a total body weight \>45 kg. * 6MWD ≥ 150 and ≤ 450. * Pre-randomization RHC documenting a minimum of PVR ≥ 400 dyn ∙sec/cm5. Key Exclusion Criteria Part B: * Any medical or psychiatric condition or laboratory abnormality. * Stopped receiving pulmonary hypertension chronic general supportive therapy 90 days prior to Day 1. * Pulmonary capillary wedge pressure \> 15 mmHg on right heart catheterization (RHC) conducted during Screening. * History of severe allergic or anaphylactic reaction or hypersensitivity to recombinant proteins or excipients in investigational product. * Major surgery within 8 weeks prior to randomization. * Participants who smoke more than 10 cigarettes (or equivalent) per day or has a smoking history ≥10 pack-years.