Since you cannot download it from official stores, you have two primary options:
Open-world web-swinging on PC/Consoles, and a unique 2D "Metroidvania" style on the Nintendo DS. Spider-Man Web of Shadows is way better on the DS! Spider-Man Web of Shadows Free Download
The most reliable way to own the game is to purchase a used physical copy for PC, Xbox 360, or PS3. Where to Find it Safely Since you cannot download it from official stores,
Because the game is no longer sold or supported by Activision, many gaming communities classify it as "abandonware." Spider-Man Web of Shadows Free Download