It should work on almost every binary.

The exception are NSIS installers, self-extracting archives (exe rar files), files with IDL interfaces.

When NSIS starts they will try to open it's own exe file and find the section in which it's packed data is stored. But UPX will remove those sections and create .UPX section with compressed data.

You can actually set NSIS to UPX-pack its stub, if you're so inclined, but the stub is pretty small already.

This isn't what I meant. If you try to run UPX on someones NSIS installer the resulting file will fail to launch.

When I first found upx I did this a couple of times only to fail pretty badly and then I stopped doing it. This was like 7-8 years ago when I first tried the portable version. Never found the cause till today.

Hmm, works for me tho