If you have Acrobat (not reader -- standard or professional), creating clickable links within PDFs is as simple as click and drag. E.g. as documented here:
http://www.webmasterworld.com/graphics_multimedia/3439118.htm
"You need the full Standard version of Acrobat or better, go to tools->Advanced Editing->Link tool. Draw a rectangle around the text to link and it will prompt you for link options (in-document, web URL, link colors, etc.)"