The Pakistan visa on arrival can be applied to the two situations:
1-You may apply, if you are a new applicant and do not have a visa.
2-On the other hand, you may have a visa but you would love to extend your stay in Pakistan. In this situation also you can apply for the visa on arrival.
There are some documents you should have while applying for a visa online. A latest colored photograph, valid passport with 2 blank pages for stamping.Invitation letter of the approved tour operator. This is for group tours. More details: What Should You Do For A Pakistan Visa On Arrival? - Big Bear Lake