You are right , .pak files have commun use . In our case Unreal Engine does package the game files in .pak files.
Please follow these :
1- Go to the main page and at the downside, there is demo package. Please download that . it is around 2 gb file and contains the basic files of the game.
2- Then copy and paste you "mod" pak file into the content folder.
After that, you may see the folder is something like this . The names might be different but you will have 2 pak files. Run the game .
Dont try to open pak files. just copy and paste if you will have issues please let me know.
Thank you