Drag and drop the .pkg file directly into the main RPCS3 window, or go to File > Install Packages/Raps and select it.
The most compelling argument for PKG/RAP files is game preservation. Digital storefronts are not eternal; the PS3’s PlayStation Store has already been threatened with closure multiple times. Countless digital-only titles, DLC packs, and patches are at risk of disappearing forever. By downloading PKG files and their corresponding RAP licenses, users can create a permanent, offline backup of their purchases and even titles that are no longer available for sale. pkg rap files ps3 better
To understand why PKG+RAP is the "better" choice, it helps to compare it to the alternatives used by the PS3 community. Drag and drop the