Google Drive is Google's file sync app that lets you store all of your files online alongside your Google Docs documents, and keep them synced with all of your devices.
File signed supplier agreements with proof in the supplier folder.
Keep signed supplier agreements and their signing proof together in the right supplier folder, so your team can find both when needed.
Name missing supplier contract files on the approval request.
Your requester sees exactly which contract files are missing, while the approval stays in correction for follow-up.