This pilot clinical trial will evaluate the initial safety, feasibility, and pharmacokinetics of microbiota transplant therapy (MTT) with antibiotic pre-conditioning and fiber supplementation vs. placebo in patients with pulmonary arterial hypertension (PAH). This trial will inform development of future trials of MTT as a treatment for PAH. 24 PAH patients will be randomized to receive either MTT with antibiotic preconditioning + fiber supplementation, MTT with antibiotic preconditioning + placebo supplementation, or placebo + placebo supplementation. MTT will in a capsule form composed of freeze-dried, encapsulated intestinal microbiota from healthy donors. Fiber supplementation will be 10-14 gm oral fiber supplement. Patients will be followed at week 1, week 2, week 4, week 12, and week 24. Patient will undergo stool sample collection at baseline, week 1, week 4, and week 12, blood sample collection at baseline, week 4, and week,12. In addition, patient will undergo an echocardiogram, six-minute walk test (6MWT) and quality of life questionnaire at baseline and at week 12.