1. A document verification method executed by an apparatus for verifying a binder document which includes a plurality of binding target documents, the method comprising: a selection step of selecting the binding target documents which are to be included in the binder document; a decision step of deciding whether each of the selected binding target documents is to be targeted by verification or not; an assigning step of assigning an identifier for each of the selected binding target documents based on the decision in the decision step, the identifier indicating whether each of the binding target documents is a verification target document or not; a storing step of storing the binder document which includes the binding target documents selected in the selection step; a determination step of determining whether each of the binding target documents included in the binder document is a verification target document or not, based on the identifier assigned for each of the binding target documents in the assigning step; a verification step of performing a verification process for each binding target document which has been determined to be a verification target document in the determination step, wherein the verification process is not performed for binding target documents which have not been determined to be a verification target document in the determination step; an integration step of integrating the results of the verification processes performed in the verification step; and a verification result storing step of storing the verification result integrated in the integration step as the result of verification of the stored binder document.