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.
Put marked proof corrections on the maker's revision checklist.
Your maker gets a clear fix checklist with reviewer marks linked to the proof revision they belong to.
Put the latest page proof on the card before checking corrections.
Your rework card points to the latest image or PDF proof, with the next reviewer clear when a shared file changes.