Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window ppc-3.3.0-v20070607-8y8eE8NEbsN3X_fjWS8HPNG.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.