From the Ask Apache blog, there's a quick tip that makes it simple to give your site's visitors the option to either download or view a PDF file no matter what their browser default is.

One of the most annoying things on the Internet for me is when I click on a link to an Adobe PDF file. For me this is annoying to the extreme because the PDF file is openened directly in your browser because of the Adobe PDF Plugin that almost all browsers have installed. [...] For me an ideal solution would be to offer me choices.

His solution uses mod_rewrite in an .htaccess file to push the visitor to a PHP file. This file grabs the filename they want and pulls in the content, pushing it back out the other side with the "attachment" header that forces a request box on the browser.