sejda
· An extendable and configurable PDF manipulation layer library. A ready to use java library to perform PDF documents manipulation without having to deal with the low level API. Sejda offers many "ready to go" manipulations implemented using the SAMBox but it can be extended to use other implementations.
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 sejda-parent-5.0.6.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.