The style and design of the document holder will determine where you should place it on your desk. Most document holders with a stand or clamp can sit in any position that is comfortable for you, either between your keyboard and monitor, next to your keyboard, or next to your screen. Make sure to place it in a position to minimise the need to turn your neck or strain your eyes while you work.
You may need to test different positions and desk layouts before finding the best placement on your desk. Other document holders are designed to slip over your keyboard so you can place your documents above your keyboard while you type, reducing the need to turn your head to the side. A desk paper stand or desk paper holder is a convenient addition to your workspace.