This is the adjective you put in front of a noun to tie that noun to the country: Pakistani food, a Pakistani passport, the Pakistani government. It covers both the place and the people, so it works for things made there, people born there, and customs that come from there. The capital letter is not optional. Words built from the name of a country keep the capital wherever they stand in the sentence, which is a habit Chinese does not have and so is easy to forget.

Here the same word is a noun, standing on its own for a person. You can count it, so it takes a and s: a Pakistani, two Pakistanis, millions of Pakistanis. The source also covers people of Pakistani descent, which means someone can be called a Pakistani because of where their family came from, not only because of where they live now. Note the shape of the word does not change between the adjective and the noun — only the job it does in the sentence changes.