You're looking for a PDF on endgame tactics by Van Perlo, specifically a 39-piece scenario. I'll provide some general information and potential resources.
: To sharpen calculation in simplified positions. van perlo 39s endgame tactics pdf better
Most books say knights are bad in rook endgames. Van Perlo shows 40 examples where a knight on the rim (e.g., Nh1 or Na8) actually sets up a devastating fork vs. the enemy king and rook. Standard theory calls this "accidental." Van Perlo calls it a "pattern." You're looking for a PDF on endgame tactics