It’s not possible to unzip files natively on the iSeries, however there is a version of PKZIP for the iSeries that will allow you unzip files on the iSeries.
The simplest answer is usually to review the Java jar tool. Note the link on that page to Sun’s documentation. Also note the reference to the Java ajar tool.
There may be particular requirements such as transferring files in binary mode, passwords on the .zip file, etc., that the creator of the file will have to help with.
PKZip/Unzip is available as a commercial and supported product. Web site is noted above.