To open pdf files you will need a .pdf reader.  Foxit PDF Reader is a free alternative to Adobe Reader, that allows you to view and print PDF files.  Unlike Adobe Reader, it opens PDF files very fast and it supports all standard features, browser integration, select/copy of text from the documents.  You can get Foxit PDF Reader at the following site:

http://www.foxitsoftware.com/foxitreader/foxitreader.zip

If you prefer the free Adobe Acrobat Reader, it can be obtained at the following address

 (it is 10x the size of the Foxit download):

http://www.adobe.com/products/acrobat/readstep2.html

(you do not need Adobe Yahoo! Toolbar or Adobe Photoshop® Album Starter Edition)