Adobe Acrobat adding background not working

I just downloaded adobe acrobat today. I'm trying to add a background but its not working. It's not the opaque settings. I've tried several different pdf's as backgrounds, so I know it's not the one im trying to use. After I select it, it says it is there, but I can't see it. I then remove the background and try again, trying all the settings with no luck. Any advice would be apprecaited!