How do you ensue required documentation is received with materials

We have some materials ordered on purchase orders where there is documentation provided by the vendor which we are required to maintain for regulatory purposes.  There is no cost to the documentation.  The documentation doesn't generally come at the same time as the materials.  We wil withhold payment on the purchase order if documentation is not received.

What is the best way in the system to track whether the documentation is received prior to payment?