Been having problems with my work computer when viewing PDFs in the browser  - takes far too long to start up Adobe Reader on my machine. I found this snippet of code to force download the PDF instead.
Upload the file you want to make available for download from your Website. For example, huge_document.pdf
Edit a new PHP file and name it the same as your file to be downloaded, only with the extension .php. For example: huge_document.php
Your PHP file should look like this:

<?php
header(‘Content-disposition: attachment; filename=huge_document.pdf’);
header(‘Content-type: application/pdf’);
readfile(‘huge_document.pdf’);
?>

Link to your PHP file as a download link. For example:

<a href=”huge_document.php”>Download my huge document (PDF)</a>

To change the MIME type of the file you want to download, edit line 3

header(‘Content-type: application/pdf’);