In WWE 2K17 and beyond they are being replaced by HSPC files. The main difference between HSPC and PAC files are that the sub files inside of HSPC files are named using string hashes that have no legible naming pattern.
.pac files are required to be installed while the game isn't running, for they are only loaded on game startup.
File Breakdown and Analysis
This file type is different for various games. This section breaks down this file type per game.
A Breakdown of this File Type in WWE 2K17.
A Breakdown of this File Type in WWE 2K16.
A Breakdown of this File Type in WWE 2K15.