This is a common issue but few people know there’s a simple HTML 5 solution:
<a href="./directory/yourfile.pdf" download="newfilename">Download the pdf</a>
Where newfilename is the suggested filename for the user to save the file. Or it will default to the filename on the serverside if you leave it empty, like this:
<a href="./directory/yourfile.pdf" download>Download the pdf</a>
Compatibility: I tested this on Firefox 21 and Iron, both worked fine. It might not work on HTML5-incompatible or outdated browsers. The only browser I tested that didn’t force download is IE…
Check compatibility here: http://caniuse.com/#feat=download