The "Flashback Database" feature is supported only at the CDB-level, which does not allow a "granular flashback" at the PDB-level.
In many cases, the requirement is to rewind one specific application (i.e. PDB), and not the entire CDB.
RMAN, for example, allows a PDB point-in-time-recovery, so the same should be in the FLASHBACK DATABASE feature.
Update: Flashback PDB is not provided with Oracle Database 12c Release 2:
You can perform a flashback database operation for a single pluggable database (PDB) in a multitenant container database (CDB) using the FLASHBACK DATABASE command.
Performing a Flashback Database operation on a particular PDB modifies only data files related to that PDB. The other PDBs in the CDB are not impacted and are available for use.
When using restore points, you can perform a flashback database operation either to a CDB restore point, PDB restore point, PDB clean restore point, or PDB guaranteed restore point.
For more information please refer to: Database Backup and Recovery User's Guide