If I download with Safari, I get PortAuthority.dmg.bz2. If I double click on that file in the Finder I get a PortAuthority.dmg.bz2.cpgz file.
bunzip2: PortAuthority.dmg.bz2: trailing garbage after EOF ignored
If I then do bunzip2 -d it gives the same error message but produces a .dmg. However, trying to mount the .dmg gives a no mountable file system error message.
Changing the setting of Safari preference "Open safe files after downloading" didn't seem to have an effect on this.
However, if I download it with Firefox I get a mountable .dmg file with a usable PortAuthrority.
-David