This is a child custody matter. If I have Legal documents stating that my ex husband, "who now has custody of my daughter," is not her biological fath

There is, unfortunately, a large body of law on this subject. First, you do not have the right to make a unilateral custody determination regarding your child; you will be in contempt of court if you do not return her as required by the existing court order and you may be committing a crime. Second, subsequent paternity tests are frequently held to be insufficient to trump an existing and longstanding parentage order; there is a strong judicial preference for parentage, once settled, to remain settled. The effect of even a judicial determination that the "real" father is "real" will establish a total stranger in that relationship; that can hardly be in the child's best interest. Again, if you act unilaterally, you do so at your peril. See an attorney.