Starting with version 4, PDFsam Basic is distributed including a simplified version of the Java runtime used to run the application. It is no longer necessary to install Java.

PDFsam Basic v3.x requires Java 8 with JavaFX installed. JavaFX is included in Oracle Java Runtime Environment, what about OpenJDK?

java.lang.NoClassDefFoundError: javafx/application/Application

Some Linux distributions have separate packages for OpenJDK and JavaFX so, if you install the OpenJDK package and try to run PDFsam 3.x, you get an error because of the missing JavaFX : 

Exception in thread "main" java.lang.NoClassDefFoundError: javafx/application/Application
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	at org.pdfsam.community.App.main(App.java:34)
Caused by: java.lang.ClassNotFoundException: javafx.application.Application
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 13 more
Solution: Install the openjfx package for your Linux distribution