I've had this weird issue for a while and have just been living with it for the time being. My PDF's open in Notepad. Logically you'd go to the "Choose default program to open this file type" area but Adobe Reader doesn't show up. When I go to "Browse" I find the Adobe Reader executable but when I click on it it doesn't show up in my default program list. So I uninstalled Reader and I repeated the process but I still have the same problem. When I launch Adobe Reader and select my PDF it opens fine so I don't think it's a matter of the program not being installed correctly.