A Study of Pembrolizumab (MK-3475) in Relapsed or Refractory Classical Hodgkin's Lymphoma (rrcHL) or Relapsed or Refractory Primary Mediastinal Large B-cell Lymphoma (rrPMBCL) (MK-3475-B68)

Inclusion Criteria: * Have a histologically confirmed diagnosis of cHL or PMBCL, according to the World Health Organization (WHO) classification \[Swerdlow, S. H., et al 2008\]. * Has radiographically measurable cHL or PMBCL disease as per Lugano classification with at least 1 nodal lesion (which has not been previously radiated) that is \>15 mm in long axis, regardless of the length of the short axis, and/or extranodal lesion of \>10 mm in long and short axis. PMBCL-Specific Disease Characteristics: * Have relapsed or refractory PMBCL and: * Have relapsed after auto-stem cell transplant (SCT) or have failed to achieve a CR or PR within 60 days of auto-SCT. Participants may have received intervening therapy after auto-SCT for relapsed or refractory disease, in which case they must have relapsed after or be refractory to their last treatment. OR \- For participants who are ineligible for auto-SCT, have received at least ≥2 lines of prior therapy and have failed to respond to or relapsed after their last line of treatment. At least 1 of the prior lines of therapy must contain a rituximab-based regimen. Note: Participants should not need urgent cytoreductive therapy. * Relapsed Disease: disease progression after achieving an overall response of PR or CR in response to the most recent therapy * Refractory Disease: failure to achieve CR or PR to the most recent therapy. cHL-Specific Disease Characteristics: * Have relapsed or refractory cHL and: * Have relapsed during the…