e-book解壓

why我download左本書之後,(jar)
睇唔到?但係人人都change到個file做java...但係我唔得?
可唔可以teach me how to解壓縮?我想要有picture...
before我解壓縮個file,佢change左做txt,我睇唔到...but before I try,it can change to be java....why???
(before someone tells that we no need to download some things to change the file..)