I would like one of my links to open a .pdf file.
How do I do this?

Hi, simply link directly to the pdf file. i.e http://www.yoursite/thepdf.pdf

As long as the user has acrobat installed it will open fine, would be a good idea to put a not on the page or near the link saying:

"This document requires Adobe Acrobat Viewer to open" Download it here http://www.adobe.com"

if you are hand coding the html - just replace the normal file name with the pdf file name e.g. <a href="\myfile.pdf"> </a>