How could I obtain an invitation letter if I know nobody in China?

What kind of visa are you going to apply for?
If it is an L visa for tourism, your travel agency will handle it for you. Since Pakistani nationals are encouraged to join a tourist group (more than 5 people), which should be arranged through a qualified local travel agency.
If it is an F visa or Z visa, your inviting organization should obtain the invitation letter for you from the authorized government organization like the local foreign affairs office, ect. in China.