Mr.Ahsen, it is not easy for pakistani applicants to get a China visa in Saudi Arabia, as I know, you need to apply for tourist visa (L visa) if you go to Shanghai to visit the Expo, however, for tourism, Pakistani citizens can't apply for individual tourist visa, you have to contact a qualified travel agency of China and settled down all the details of the tour. And then the tourist agency may arrange a visa notice form for a travel group (more than 5 people) and a group-visa for you.

It is impossible for you to get an official government letter if you don't have a business partner in China, or you are not invited to come to China by an organization, unit or government office.