I have just downloaded an ISO of the Windows 8 Operating System onto my PC that is currently running Windows 7. Instead of burning a DVD, how can I install Windows 8 directly? Also, it is very important that all of my documents are not destroyed. Will this keep all my documents?
In that case: Step 1 in to make a backup of all of your documents.
Seriously. If you are worried about that, make and test a backup. And store it somewhere else. Even if the upgrade goes well you want it for the day something goes wrong.
Having said that: If you want to upgrade from Win 7 (normal 64 bit version) to windows 8 (normal 64 bit version) then that should work without problems.
If you try to change between 32 bit and 64 bit versions then you will need a clean installation.