do i need to show invitation letter at the immigration of Beijing airport?

Hi.i got ''F'' chinese visa through travel agent . duration of stay is 30 days. my purpose of visit infact is to visit china , but as it is very difficult for Pakistan nationals to get visit visa for china , so my travel agent got me ''F'' Business visa from chinese embassy in islamabad but i don;t have any invitation letter of any sort or from any company which supports that im traveling china for business. so i want to ask does the immigration depart. at beijing airport ask for invitation letter. or they just let u go through on the base of visa. any one can tell plz?Thanks