Back before Forza Horizon 3 released, Microsoft Studios advised that a PC demo of the game would become available sometime after launch.
If you go to the Window store, the demo is now available to download and play. It includes a sample of the Horizon Festival, and like the Xbox One demo, has a few races for you to enjoy as well as access to a limited portion of the map. Also within the demo is a small taster of the management aspect of the festival.
The download size for the demo is 20GB, with the system requirements also available on the page.
Source: God is Geek