This study will have two parts as follows:

The PK Lead-in Phase of the study will evaluate the steady state pharmacokinetics (PK) and confirm the dose of ledipasvir/sofobuvir (LDV/SOF) fixed dose combination (FDC) in hepatitis C virus (HCV)-infected pediatric participants. The PK Lead-in Phase will also evaluate the safety, tolerability, and antiviral activity of 10 days of dosing of LDV/SOF FDC in HCV-infected pediatric participants.

The Treatment Phase will be initiated by age cohort after confirmation of age-appropriate LDV/SOF FDC dosage levels. Participants from the PK Lead-in Phase will immediately rollover into the Treatment Phase with no interruption of study drug administration. The Treatment Phase will evaluate the antiviral efficacy, safety, and tolerability of LDV/SOF FDC +/- Ribavirin (RBV) for 12 or 24 weeks in pediatric participants with HCV.