If you're on Windows, use a COM object to extract the text from the Word document, and then use one of the various PDF classes (FPDF, or the built-in PDF support) to create the PDF document.
If you're on *nix, extract the text with a command-line utility like antiword, wvtext, or catdoc (Google for more information on these).
http://us.php.net/com
http://us.php.net/manual/en/class.com.php (there's an MS Word example if you scroll down)
http://us.php.net/pdf
http://www.fpdf.org/