A bstract UPX is copyrighted software distributed under the terms of the GNU General Public License , with special exceptions granting the free usage for commercial programs as stated in the UPX License Agreement . UPX is a free, portable, extendable, high-performance executable packer for several different executable formats. It achieves an excellent compression ratio and offers very fast decompression . Your executables suffer no memory overhead or other drawbacks. UPX aims to be Commercial Quality Freeware .
All versions are functionally equivalent, i.e. each version can handle all executable formats.
Windows users should prefer the Win32 version because of better long filename support. Just in case, here is an archive of old versions . There will be hidden bugs. There's no reason to download the unstable versions unless you want to try the new formats. Also, a checksum of both the compressed and uncompressed file is maintained internally.