iPhone window.open(url, '_blank') does not

Question

I'm sending users to some .pdfs in a standalone web app (saved to home page, no PhoneGap) on iPhone/iPad. With window.open(url, '_blank'); these documents open on top of the web app and since there is no back button there is no way to get back. Can I open these in a new window in JavaScript, or do I just need to use a real a href?