I have run PAF5 from a flash drive for about 4 years. I find this handy when I am away from home and need to use PAF5 on a computer that doesn't have it installed.
Recently, I had a situation where PAF4 was needed to convert some PAF2 data files. Guess what? My new computer doesn't have PAF4 installed. So I plugged my flash drive into my old computer and copied the PAF4 folder to the flash drive. Now I can run either PAF4 or PAF5 from the flash drive ... or both at the same time.