Any "GTA V PPSSPP ISO" you find online, especially those claiming to be highly compressed (e.g., 300MB), is generally one of two things:

If you want an authentic GTA experience on your phone via the PPSSPP emulator, here are the real ISOs you should look for (you must own the original game or dump your own UMD):

A 7z compressed ISO file is a type of archive that contains the game's data, compressed using the 7-Zip algorithm. This compression reduces the file size, making it easier to distribute and download. However, the compressed file must be extracted and decrypted before it can be used with PPSSPP.