File with a PAC extensions contains data used in automatic configuration of a web browser in order to use a proxy, a JavaScript with FindProxyForURL function which defines rules for choosing a server. They are stored in text form, therefore they can be edited even in a simple text editor, however, they are usually located on a server and have a default name – proxy.PAC.

PAC files are usually utilised within corporations in order to control access to web sites and to balance data flow within a network. When used in a browser, their function covers providing a URL address of a file for proxy settings and initiating Proxy Auto Discovery (WPAD) protocol. PAC script, after being executed, returns an address of one or several proxies.