A Pharmacokinetics Study for Pediatric Participants With Pulmonary Arterial Hypertension

During Period I, tadalafil will be administered orally, once daily, at a low dose for approximately 5 weeks followed by a high dose for approximately 5 weeks. Dose levels are calculated based on body weight cohorts. Heavy weight cohort \>=40 kg, middle weight cohort \>=25 kg to \<40 kg. Light weight cohort\<25 kg. Participants who complete Period 1 may continue taking tadalafil in Period 2 for at least 2 years. Starting dose will not exceed the maximum weight range dose established in Period 1 and after the first 3 months of Period 2, the dose may be adjusted based on available safety and efficacy information.