Switching spaces when opening a new app

If I want to, for example, open a PDF from the internet, it will open in another window which already has preview running. This is really annoying. Especially since I am using 16 windows right now to juggle my dissertation and computer simulations. I didn finally realize that I can assign certain applications to open in certain spaces, which is quite helpful- I usually move firefox to space 4, itunes to space 8, etc. But i haven't figured out how to fix this other issue, even though I have unchecked the box in system preferences that says "When switching to an application, switch to a space with open windows for the application."