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 ledger-api-protos-100.13.56-snapshot.20200331.3729.0.b43b8d86.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.