The limitation of PKG files is that they are encrypted. The file is a tiny license key that acts as a secure container to decrypt these files.
RAP files are the license activation files that enable decryption of PKG content. When you purchase a game on the PlayStation Store, you receive both the encrypted PKG (the content) and an associated RAP (your license to access it). These files are identified by their content ID naming convention (e.g., UP0177-NPUB30625_00-VIRTUAFIGHTER5FS.rap ), which links them to specific titles. pkg rap files ps3 better
If you prefer to avoid a USB stick or if your console lacks specific USB ports, you can use a file manager: The limitation of PKG files is that they are encrypted