A file (extension .PBP ) stands for "PlayStation Portable Binary" or "PBPack" . It was originally designed by Sony as a container format for downloadable PlayStation 1 games (called "PSOne Classics") to run on the PlayStation Portable (PSP) via its built-in PS1 emulator (POPS).
If you are building a new PS1 collection for PC or modern handhelds, consider using instead. But for PSP, PS Vita, or a nostalgia-driven archive, PBP is still perfectly viable. ps1 pbp roms archive