Pennsylvania does have a permit process for state parks and forests.

Geocaching in Pennsylvania State Parks and Forests
Guidelines for Placing Geocaches (and letterboxes)

It is the geocaching form, but it has a checkbox for letterboxes as well. Simply attach your clues and also a map marking where you plan to place your boxes and turn it into the park manager. It should take about a week or two for them to approve or deny the permit. If approved you will be given a sticker to place on the box to mark it has been permitted. The permit is good for three years.