PAK

PAK file formats are package formats previously used by GoldSrc (and many other 3rd party engines like Unreal Engine 4 and 5) engine games to store content. It was first introduced in Quake which GoldSrc subsequently adopted it.

This was later changed during the WON/Retail to Steam transition throughout 2003-2004, which the game file is now stored inside .gcf file, then later in 2013, with SteamPipe, the game file is now stored directly to the filesystem (example: ...\SteamApps\common\Half-Life\valve), unlike Source and Source 2 engines (which continued to packing game files inside .vpk file) as most modern systems are fast enough to load GoldSrc game files quickly.

As of today, the Steam version of Half-Life, along with other GoldSrc games are still able to load the .pak file.