PAK
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PAK file formats are package formats previously used by 
GoldSrc engine games to store content. It was first introduced in 
Quake engine which GoldSrc subsequently adopted it. Unlike Quake, GoldSrc prioritizes loose files over those found in a PAK.
This was changed during the WON/Retail to Steam transition in 2003, when the game files were stored inside GCF files located in 
...\Steam\steamapps\username\ folder. Since the 2013 SteamPipe update, the game file have been stored directly to the filesystem (example: ...\SteamApps\common\Half-Life\valve), as opposed to Source and Source 2 engines (which continued to packing game files inside multiple VPK files after SteamPipe transition) 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 despite longer being used to store vanilla game files.
Important:PAK is often used as a generic file extension in the industry. PAK files used by other software, such as Chromium, Unreal Engine 4+, or CryEngine are completely unrelated to the file format used by GoldSrc and id Tech 2.
Note:MP3 files cannot be packaged in a PAK file. 
Confirm:Structs
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C code from 
common/qfiles.h in the original Quake II development tools:
#define IDPAKHEADER (('K'<<24)+('C'<<16)+('A'<<8)+'P')
typedef struct
{
char name[56];
int filepos, filelen;
} dpackfile_t;
typedef struct
{
int ident; // == IDPAKHEADER
int dirofs;
int dirlen;
} dpackheader_t;
#define MAX_FILES_IN_PACK 4096
Important:MAX_FILES_IN_PACK is an engine limit, not a format one. The 2023 Kex remaster of 
Quake II exceeds MAX_FILES_IN_PACK, and as such will not load in some tools such as 
PakScape.
