In some rare cases, particularly with certain PSP homebrew or non-encrypted files, simply changing the file extension from .pbp to .iso might work, though this is for commercial PS1 EBOOTs as they require actual extraction.
While PBP files work natively on PSP hardware, converting them to ISO offers several advantages: convert pbp to iso
You can look into the POP-FE project , which uses Python scripts to automate Eboot creation and could be reversed for extraction. In some rare cases, particularly with certain PSP