The link I gave also shows you a program called Wally, and another one called Half-Life Model Viewer. Both can browse the .pak file, allowing you to see what files are inside. Both can extract files too although Wally is generally easier to work with, as it also allows you to create your own .pak files and such.
Note that .pak files are just a way of storing other files. Look at it as a sort of box in wich you would put your drawings, so you don't loose them somewhere in your room...