// Declare your shortcode
add_shortcode("download", "downloadFunction");

// Second Declare your shortcode function
function downloadFunction($att, $content, $code){

    // your function is passed 3 Arguments
    // $atts : this is an array of the shortcode\'s attributes
    // $content : this is the content between the code. in your case the file url.
    // $code : this is the code used. in this case it will be download.
    // here we only need the $content of the shortcode which we place for the file

$return = \'<a class="download" href="\'.$content.\'">download file</a>\';
$return .= \'<a class="download" href="http://docs.google.com/viewer?embedded=true&url=\'.$content.\'">preview file</a>\';

// we then return the result.
return $return;
}