I am a medical student and i read a lot of medical books in adobe reader. Each book is around 1 GB. I read in my ipad or iphone while travel or when im at ward. When i am at my room i read from my mac pro or macbook. If i need to synchronise the highlighted/underlined/annotated portions of any pdf accross all those devices, the entire pdf need to be synced which accounts to a huge amount of data per day. It is better if there is an option to synchronise annotations alone.