Unlike video games, tabletop rules often get errata and updates. The PDF community allows for revised versions of the Brigade rules to circulate, fixing ambiguous wording or rebalancing overpowered artillery batteries.
Here is where the search for the Brigade Fire and Fury PDF gets complicated. Because the rules are technically still intellectual property (IP) owned by Fire and Fury Games, they are not legally available for free on typical "PDF aggregate" sites. brigade fire and fury pdf
However, the wargaming community walks a fine line: Unlike video games, tabletop rules often get errata
If you are looking for a legitimate copy of the Brigade Fire and Fury PDF, your best bets are: If you are looking for a legitimate copy
Just because you cannot find the PDF instantly does not mean you cannot play the game.