Jquery snippet to add target="_blank" to all external links in a documen and force PDFs to open in a new window/tab as well.Replace mydomainname with yours to skip internal links.

external-link-new-tab.js

 $(document).ready(function(){
	
   // external links to new window
    $('a[href^="http://"]').not('a[href*="mydomainname"]').attr('target','_blank');
    
    // force PDF Files to open in new window
    $('a[href$=".pdf"]').attr('target', '_blank');

  });

Hello! Thanks for gist. A small correction: mydomainname must be in quotes, like:
not('a[href*="mydomainname"]')