Most papers I read are PDF documents, and I need to read two or three at the same time to compare different approaches related to the same topic. It is luck that my boss put two LCD's on my desk. Adobe reader annoyed me at first because I could not make two documents displayed on different monitors. I found the trick is at Window->Tile. It is perfect for two documents, but things will be messy when you have more than four documents opened at the same time. Now it is time to use gsview to read as many documents as you like. The difference between adobe reader and gsview is that the former can only have one application instance running on the system, but the latter has no such restriction. Therefore, there is no way to display different parts of one document on two monitors at the same time by using adobe reader when you do not make a copy of that document.