0x00-0x03 Just a header. "PACK" or 0x50 0x41 0x43 0x4b
0x04-0x07 The 32-bit offset to the file entries from the beginning of the .PAK file.
0x08-0x0b The length of the entries. Each entry is 0x40 chars (bytes) long, so this corresponds to length/0x40 entries.
Each entry consists of:
0x00-0x37 0x38 chars (bytes) for filename w/ path. Note that this is the path from the directory your quake executable lives in, AND that the directory separators are / rather than \. The path is null-terminated.