3D FlipBook - Caching Issue

Hi there, I have the pro version and I've updated the plugin recently. When I upload our new PDF to an existing Book, our client doesn't see the updated version. They still see the old version even after we delete it from the server. We've had them clear their cache, try Incognito, different browsers (they use Chrome), flush their dns on their computer but nothing works. We're not seeing the updated PDF version. Please advise.