We have recently reported a new mechanism of colossal magnetoresistance (CMR) in electron doped manganese oxypnictides NdMnAsO1-xFx. Magnetoresistances of up to -95% at 3 K have been observed. Here we Show that upon replacing Nd for Pr, the CMR is surprisingly no longer present. Instead a sizable negative magnetoresistance is observed for PrMnAsO0.95F0.05 below 35 K (MR7T (12 K) = -13.4% for:PrMnAsO0.95F0.05). A detailed neutron and synchrotron X-ray diffraction Study of PrMnAsO0.95F0.05 has been performed, which shows that a structural transition, T-s, occurs at 35 K from tetragonal P4/nmm to orthorhombic Pmmn symmetry. The structural transition is driven by the Pr 4f electrons degrees of freedom. The sizable -MR observed below the transition most likely arises due to a reduction in magnetic and/or multipolar scattering upon application of a magnetic field.