If you don't want the shadows of the papers to continue to the background layer then turn off Accepts Shadows on the background. I think this would look odd though.
The other option would be to pre-compose your papers layers, make the pre-comp 3D and turn off casts shadows.
I actually don't think I understand what you are trying to do.
Ok, here's the deal. I have a project that is a 3D object: a pile of papers (actually it's a notebook). You can see a part of it in the image. I don't want it to have backround at all. In the image I made it just to show the shadows, so that's why I cant turn off cast shadows. As you can see the shadows on the backround are not realistic because it makes a shadow from every page separately.
And I have another project into which I want to transfer this 3D notebook. So I want this notebook to transfer into another project without shadows so I can make a same shadow for the whole notebook. That's why I asked if I could crop the shadows away from the first projects backround.
The pages of notebook are moving (notebook is opening). I think I could solve this problem if i fade in or move the shadow angle a bit but I don't want to do that if there's easier way to complitetly remove the shadows from backround (I don't have a backround colour, the blue colour is just to show you the shadows).
I'm not understanding how you created the 3D object but I'd make another copy without a drop shadow, and if it's multiple layers, pre-compose them and just eh pre-comp as a track matte for the original.