Is it possible to reassemble documents that have been sliced and diced by a shredder? It depends on the type of shredder used.
Some more expensive shredders leave paper in smaller pieces because they cut horizontally and vertically. Those pieces are very difficult, but not impossible, to reassemble. Older model machines shred paper into long thin strips.
Inside Edition staffers tried to get an idea of what FBI agents are facing as they attempt to reassemble the Michael Cohen evidence.