Is the marriage to a woman who claimed that her former non-Muslim husband had divorced her, but it was later found that she had not received a divorce document, considered an invalid marriage?

If the wife converts to Islam while her non-Muslim husband remains in his religion, the marriage becomes null and void. It is permissible to marry her after her waiting period (iddah) expires, which is by giving birth for a pregnant woman, or by one menstrual cycle according to the preponderant opinion, or three menstrual cycles according to another opinion.

If the marriage takes place after the waiting period, with the permission of her Muslim guardian, and in the presence of witnesses (or the Islamic center acting as guardian), then it is a valid marriage. What she concealed from you does not harm the validity.

If the marriage took place before the waiting period, it is void, and she must observe a waiting period from you and renew the contract in the correct manner. If she has no guardian, the Muslim judge or whoever acts in his place may marry her, or she may appoint a just Muslim man as her representative.