PDFsam (PDF Split And Merge)
Official website pdfsam.org
PDFsam Basic is open source under the GNU Affero General Public License since version 3, previous versions are released under GPLv2.
PDFsam Basic is written using JavaFX. Version 4 is released as a self-contained application and includes a jlinked OpenJDK 11 runtime while version 3 requires a Java Runtime Environment 8 with JavaFx installed in order to run.
This is a simple guide that can help you building PDFsam Basic
Contributes are more then welcome, just please make sure you first read the contributing guidelines
If you want to contribute translating PDFsam to your language you can do it here
Tips and tweaks
A list of properties and arguments that can tweak PDFsam behavior can be found here