about invitataion letter

If the Chinese company who sent you invitation letter is a private enterprise, ask then to apply for an confirmation of invitation from the local foreign affairs office for you again.

You should apply for a China F business visa with the documents below:
1. Your passport with over 6 momths validity and more than 2 blank pages;
2. a completely China visa application form affixed with a passport-size photo at the appionted place;
3. invitation letter from the company & confirmation of invitation from the local foreign affairs office;
4. round-trip air ticket
I know that there is Emabssy of China in Islamabad and a China consulate in Karachi? You may just apply for the visa from one of them that near to your living place.