web service or shared dll ?

Hi all,
I need to develop a method who will be call by another application  (asp net web site)..
This method will pass a document file (input file) and some other information ..

My head of project  tell me do to a method in project A (asp net web site) and add the dll A to the project B (asp net web site).
Ok, but i wonder on this sunday afternoon :) why not use web service ... ?

Anyone tell me if it's better to use shared dll or web service ?

Thanks for your time and knowledge