What seems to be the purpose of this document that was written by William Penn?

I'm assuming you mean the one that goes a bit like this, "II. The air is sweet and clear ... Your kind cordial friend, William Penn". This document was a letter from William Penn, proprietary and governor of Pennsylvania in America, to the committee of the free society of traders of that province, residing in London. The purpose of this letter was likely to encourage people to move to Pennsylvania.