The purpose of the study is to learn how the study medicine called PF-07868489 is tolerated and acts in healthy adult people and people with pulmonary arterial hypertension (PAH). Part A: An investigator- and participant-blind, sponsor-open, placebo-controlled, single ascending dose study to assess the safety, tolerability, and pharmacokinetics (PK) of PF-07868489 in healthy adult participants. Part B: A 24-week, randomized, double blind, placebo-controlled study to assess the safety, tolerability, PK, and pharmacodynamics (PD) of PF-07868489 in adult participants with PAH.