If you don’t mind all PDF attachments been forced to be downloaded then you can use something like this:
<?php
if (have_posts()) : while (have_posts()) : the_post();
$pdf_title = $post->post_title;
$uploads_dir = wp_upload_dir();
$attachment_src = get_post_meta( $post->ID, '_wp_attached_file', true );
$pdf_src = path_join( $uploads_dir['basedir'], $attachment_src );
header("Pragma: public"); // required
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: private",false); // required for certain browsers
header("Content-Type: application/pdf");
header("Content-Disposition: attachment; filename=\"".$pdf_title."\";" );
header("Content-Transfer-Encoding: binary");
header("Content-Length: ".filesize($pdf_src));
ob_clean();
flush();
readfile($pdf_src);
endwhile; endif;
?>
Put the above code in a file called pdf.php in your current theme folder. Then instead of linking direct to your pdf (http://example.com/wp-content/uploads/2011/01/Guide-to-Owning-a-Listed-Building.pdf), link to the attachment URL: (http://example.com/help-and-advice/attachment/guide-to-owning-a-listed-building/)
Doing like the above you can edit the code to do other fancy stuff such as track downloads and add some level of authentication while protecting the actual location of your PDF’s.