Evidence - cross examine or Claim as my own.

I am building a Defense as a Pro Se litigant in a civil trial. In the complaint, the Plaintiff introduced a document which unbeknownst to them (but not me) has damaging evidence to their case. I believe when I point out specific details in this document, it would be enough to both exonerate me and result in judgment for my counterclaim. With that said, am I obligated to cite the same document as my own evidence when I produce my Answer and Counter, or can I be vague and claim to have a document containing details that contradict the Plaintiff's claims, and then bring it to light at the appearance hearing.