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.
Store signed supplier terms with the completion certificate.
Keep signed supplier agreements and their signing proof together in the right supplier folder, so your team can find both when needed.
List missing supplier contract files on the original approval request.
Your requester sees exactly which contract files are missing, while the approval stays in correction for follow-up.