Stockfish is a free and open-source UCI chess engine, available for various platforms including IOS, Ubuntu, MAc, Windows etc. It is developed by Marco Costalba, Joona Kiiski, Gary Linscott and Tord Romstad, with many contributions from a GIT community of open-source developers .
Stockfish is almost always ranked first or near the top of most Eengins rating lists and is the strongest open-source chess engine in the world.It won the unofficial world computer chess championships in 2014 and 2016. Stockfish is derived from Glaurung, an open-source engine by Romstad.
More information @: https://en.wikipedia.org/wiki/Stockfish_(chess)
If you need to download it from the source please use following link: StockFish
Please note you can use it only with a GUI otherwise it is only 2kB executable code that run on silicon!, Best Free GUI, again open source with capability to review pgn files and do some database searches is SCID, You can download it from this link: SCID
Enjoy playing chess !!