Cannot navigate anymore after file opens (added app to homescreen)

I've added Outlook Office 365 Web url (for example) to my springboard. Everything works fine, because they added navigation on the web pages itself.

However when I open an attachment, it will open full screen. I have no navigation buttons left.

How can I navigate back instead of going to springboard and opening the app again? Iphone does not have a "back"button as most Androids have.