On Steam, there is a "Game Manual" DLC, which is described as follows:

"Containing all the essential information on the game including basic controls, flight and weapon control as well as understanding the interface and player stats."

On GOG, there is no such DLC (the "Game Guide" DLC, which is also available on Steam, is something different). According to GOG Database (www.gogdb.org), the game manual is not included as a goodie/download with the game, either.