I am sure the FBI has a computer program that could scan thousands of shredded pieces and reassemble them back into the original form.
I have a friend who used to work for an outfit that was developing such technology. Basically you could place the residue pieces on a flat table and an computer controlled camera would take pictures of each piece. Then the computer program would reassemble the document from the fragments.
That was quite a few years ago and I’m sure the technology has improved since then.
Of course if you incinerate your materials, there’s no way to reconstitute a document from ash.