A paper knife is a tool used by bookbinders, or perhaps by the branch of that trade known as the Collaters, to cut papers in the preparation of making books. I use my pocket sliding knife for the job ~ because it is always within reach, ready for use [12****]. If I plan ahead I will retrieve my cobbler's knife from the tool box and use that [126091].
If I want to open a letter I usually, again, reach into my pocket for the sliding knife. The paper knife would have been used in the past ~ or letter opener asit would then be more correctly called.
Good quality papers ~ especially hand made rag papers ~ can be too strong for a blunt paperknife to cut or tear them. If I plan to do a great deal of slicing papers in this way I use a cobbler's knife [126091].