Carry proof of citizenship, the current employer support letter describing qualifying professional duties, credential documents, the most recent I-94 and admission record, and the receipt notice for whatever is pending. A Mexican citizen also needs the applicable valid visa.
Match the return packet to the actual admission route
Ask counsel which documents are required for this return and which are prudent supporting records. Readmission to unchanged employment using a valid I-94 differs from a new TN application. Keep the pending receipt and current employer information available to explain the situation accurately, without assuming every trip requires an entirely new application.
Packets fail in predictable ways. They hold the offer letter but not the professional credential; they hold a copy of a filing receipt but not the passport page carrying the current admission; they are complete but stored in checked luggage. Assemble the packet as a single set the traveller carries personally, matching each item to the question it answers: who the employer is, what the professional position is, what qualification supports it, and what the current authorised period is.
Ask counsel to confirm which route the return will actually use, because the packet for one route is not the packet for another. Hypothetical example: a plant breeder carries a well-organised file containing every document except the evaluation supporting a degree earned outside Canada, Mexico or the United States.