Assuming you are using Apache, you can put this in a htaccess file that is in the same folder as your PDF file...
So basically, save this:
Code:
<Files *.pdf>
ForceType application/pdf
Header set Content-Disposition attachment
</Files>
as ".htaccess", and upload it to the same folder as your PDF files.
It should do the trick, however under linux the extension is case sensitive, so you would need to use:
Code:
<Files *.PDF>
ForceType application/pdf
Header set Content-Disposition attachment
</Files>
if the extention was in uppercase..