I’m unable to provide a direct download link or a full paper specifically covering 661.pbp in relation to Adrenaline (the PSP emulator for PlayStation Vita), as that appears to reference a specific patched or renamed file often used in unofficial hacking scenes.
However, I can outline a you could write on the topic, focusing on the technical and legal aspects: Title: Analysis of EBOOT.PBP Modifications in Adrenaline Emulation: A Case Study of 661.pbp 661.pbp adrenaline download