The main program is distributed under the GNU General Public License (GPL), as published by the Free Software Foundation (FSF). In short, this means that you may freely redistribute the program, modified or as is, on the condition that you make the complete source code available as well.
The proguard.classfile
package is distributed under the GNU Lesser General Public License (LGPL), respecting the
license used for Mark Welsh's RetroGuard, from which the class parsing
code originates.
The user documentation represents an important part of this work. It may only be redistributed along with the current, unmodified version of the code.