Good web browsers allow you to save a web page to your hard drive. Since a web page is often made from many separate files (HTML, images, stylesheets, JavaScript, Flash), for the saved page to be useful, the browser needs to stash all of those pieces somehow, and automatically edit the HTML to use the stashed pieces instead of fetching the pieces from the web site. Some browsers (IE, Firefox) can save a web page as a collection of files, but this can be cumbersome to store and send via e-mail. Some browsers (IE, Safari) can save a web page as a single file (or something that can be manipulated like a single file), but the file is usually in a proprietary format that requires the same browser to view. When tossing information around to other people, some people resort to PDFs as a way to keep text and images together in a single file.
Unipage Unifier is a free utility that saves a web page, and all its bits, as a single file that can be viewed with any web browser. It supports JavaScript functionality (not available in a PDF), and even embedded Flash animations.