Since there is no official ISO, the "RDR2 PPSSPP" downloads you see are typically creative fan-made projects:

If you see gameplay of RDR2 running on a PSP emulator, you are likely looking at one of three things: Total Conversion Mods: Many creators use Red Dead Revolver

The PPSSPP emulator is designed exclusively to replicate the hardware of the PlayStation Portable (released in 2004). Red Dead Redemption 2 is an eighth-generation game requiring massive processing power, high-end graphics capabilities, and substantial RAM—specifications that the PSP hardware never possessed. Therefore, there is no official ISO file for RDR2 that works on a PSP or the PPSSPP app.