Any file claiming to be a "PC Version" or "PC Port" of Forza Horizon 1 is unofficial.
Since you cannot buy a digital copy anymore (it was delisted in 2016), you will need a digital backup (ISO or XEX file) of the game. Forza Horizon Full PC Emulation Guide | Xenia Forza Horizon 1 Pc Download Mediafire
The only legitimate way to experience FH1 on PC is through the Xenia Emulator , an open-source Xbox 360 emulator that has made massive strides in recent years. How to Play Forza Horizon 1 on PC (The Safe Way) Any file claiming to be a "PC Version"