Downloading a "Hollow Knight PSP ISO" is highly recommended against. The file technically does not exist in a playable form for the PSP, and attempting to download one puts your device and personal data at risk. If you want to experience Hallownest on the go, the official Mobile port or the Nintendo Switch version are your best options.
Hollow Knight is available on modern platforms that can handle its system requirements. If you want to play the game on a portable device, consider these legitimate options: hollow knight psp iso