A PDF field open in a new window of Drupal
If you're looking for a quick fix for ALL pdf links on your site you could use a little bit of javascript to accomplish this as well.  This snippet finds all URLs ending in .pdf and sets the target attribute to be _blank (new window).

(function ($) {
    $(document).ready(function() {
        // Open PDF In New Window
        $('a[href$=".pdf"]').attr('target', '_blank');
    });

}(jQuery));


Since you're looking for just files of your field type you could certainly use a less generic selector, such as .field-name-field_pdf a[href$=".pdf"].  Best of luck!