OneDrive, Microsoft's file sync app formerly known as SkyDrive, lets you store your files online, edit Office documents in the free Office Web Apps, and easily access your online files from your PC.
Email the current proof to its reviewer and note the revision sent.
Your reviewer gets the current image or PDF and a clear question. Your review card shows which revision you sent.
Keep the revised proof on the task card for the next reviewer.
Your rework card points to the latest image or PDF proof, with the next reviewer clear when a shared file changes.