Panini-Pro for Mac is distributed as a mountable disk image (.dmg) file.  After it is downloaded, it may appear on the desktop as a pseudo-disk, or you may have to drag it to the desktop from your downloads directory.  Run  Panini-Pro direct from the pseudo-disk, rather than copying it to applications, since you will be replacing it often.  Dragging the desktop disk to the trash dismounts it but does not delete the .dmg file from your downloads directory.

To remove a version just delete the .dmg file from the downloads directory.

There is some documentation on the .dmg in docs/, but best to check the website for updates.