If you're referring to an MTA Bus, the M20 and M34A-SBS both travel from Penn Station to Port Authority: The boarding stop is 34th And 8th for and 41st and 8th for the drop off.
Can you be more specific; can you provide a city or town in Pennsylvania? Is this a question about a specific trip or a general (and very broad) question? What day of the week will you be departing?
I do not recall of buses departing from Penn Station New York going to Philadelphia. There is a MegaBus that departs a few blocks away from Penn Station New York to Philadelphia, and a New Jersey Transit train departing from within Penn Station New York to Philadelphia. Is that what you meant?
Thanks! Your answer is awaiting moderation.