First with a text editor (wordpad / notepad) since its contents is "human readable" (ASCII).
PGN stands for "Portable Game Notations"
If you want to use a program to read them you may use MailChess (http://www.cmpgo.de/chess/engl/indexe.html) or ECTool (http://webs.ono.com/a.valverde/ectool.htm).
Wikipedia is always a good resource for information like this. It links to a couple of online PGN viewers: http://en.wikipedia.org/wiki/Pgn
I use Chesspad when I want a freeware, lightweight PGN viewer.
brothers and sisters, how can i read and open PGN files? thanks..