PDF files are or the Adobe Acrobat format and can't be viewed from within Notepad. It's not encrypted, so to speak, it's just not plain text. Like trying to open an image with notepad.
For some reason your computer's associated PDF files with Notepad. (Associations tell a computer what program is used to open what type of file.)
If you don't have Acrobat installed, or if you're not sure if you do, visit
www.adobe.com and download the free reader. After installing it should associate PDF files with Acrobat, and you'll be able to view the file.
