File with a .PACK extension contains a compressed Java JAR archive. It may also contain the .PACK.GZ or .JAR.PACK extensions. It enables to reduce size of JAR files even up to 60%, due to which, it is easier to share them.

PACK files are created in the Pack200 tool, added to the Java 1.5.0, which enables also JAR unpacking. They are used by Java Web Starts to facilitate faster acquiring of JAR elements of a JAVA application, initiated within web browser.