This trial is testing a medication called seralutinib to see if it can help people with a specific type of lung disease (PAH) improve their ability to exercise. The medication works by blocking signals that cause the blood vessels in the lungs to tighten and narrow. Seralutinib is delivered via inhalation and is being developed for patients with pulmonary arterial hypertension. The goal is to see if this can reduce lung pressure and delay worsening of the disease.