I now know the reason for the crash. It looks like there is a bug report there too on fedora bugzilla.
My X server always crashes when I am scrolling through a large PDF file. After some time I get thrown to the login screen and all my applications get closed.
It might be linked to the radeon driver but I am not sure...
I will try if this happens with Adobe Reader too. If yes, then I will have to go back to ubuntu soon because I have an important homework to do for my studies
So be careful if you are scrolling a big PDF while doing work...
Edit: Adobe Reader seems to be safe for me. I couldn't reproduce the crash with it