this is because the browser will open the document in an acrobat plugin for your browser. there is a solution, but i don't know whether it's an html one. i know you can do it in PHP. you might have to link to say 'download_pdf.php':
Code:
<?php
$file = "yourfile.pdf";
header("Content-Disposition: attached");
header("Location: {$file}");
?>
not sure if thats the exact code but if you are interested i will dig out the proper code for you