Be extremely cautious when clicking "Mediafire" links from YouTube descriptions or unverified blogs. Many of these files can contain malware or intrusive "human verification" ads. Always check the comments and ensure the file size matches what you'd expect for a PSP game (usually between 500MB and 1.5GB).
If you find Mediafire links claiming to offer a GTA San Andreas Definitive Edition ISO for PPSSPP, they are typically one of the following: Gta San Andreas Definitive Edition Ppsspp Download Mediafire