There is because the game was never released for the PlayStation Portable (PSP) platform. Files claiming to be "GTA 5 for Android" in PPSSPP format are typically modded versions of older GTA titles or potential security risks.
Visit your chosen reputable source and download the "GTA 5 PPSSPP" ISO ZIP file. The file size will typically be between 400MB and 1GB, so ensure you have a stable internet connection. These downloads are usually in .zip or .rar format to save space and reduce download time. Gta 5 Ppsspp Download Iso Zip File For Android