Bill, if has an extension of ".paf" someone has sent you a data file from their PAF program. It could be either from PAF 4 or PAF 5. I presume you have tried opening it from the e-mail in the normal way? If you have PAF 5 installed, it should open, but if not, you can copy the ".paf" file into your DATA folder (where you have your current data). Then just start your PAF program, click on FILE|OPEN and select this new file. It should open.
On the other hand, if you only have PAF 4 and this is data from PAF 5 you will have to install PAF 5 as it wont recognize this data.