Hi,
.ppr reports are specifically for Cognos 7 PowerPlay. Cognos 8 does not use the same architecture or formats - a Cognos 8 "report" is held as an XML specification in the Cognos 8 Content Store database, not as a ppr or ppx or imr file on disk. The only options you have at the moment are to use a migration kit to move your PowerPlay reports across to Cognos 8 or rebuild them from scratch in one of the Cognos 8 studios.
Regards,
MF.