This is a very interesting query..
The documents can be sought by accused under S.91, before the cross examination of prosecution witness or after the cross examination.
If it is brought before the cross examination then the accused can get it marked as defense exhibit at the time of cross examination, there is no ambiguity on this.
But if it is brought after the cross examination and accused or defense wishes to bring forward any witness, then these documents can be referred in this defense witness chief.
The problem comes when these documents are sought after the prosecution cross is closed and there is no specific witness from the defense side. In that case accused has two options...
1. Call the prosecution witness for re cross examination under S.311 and confront and mark these documents as defense.
2. Apply to the court that these documents are received from the prosecution under instruction from the court (highlighting their defense values over the cross examination)..and pray for their marking as defense documents.
It should not be forgotten that such documents are summoned by the court under S.91 (suo moto or on application), so it becomes the part of record and reference can be made in the argument to counter the otherwise available evidence of the prosecution. The court cannot and will not seek any specific marking of such documents, because it tantamounts to admission of fact by complainant if he is bringing it under court's order.