Legal Question in Landlord & Tenant Law in Pennsylvania

No. The only reason you can withhold payment of rent is if the landlord breaches the warranty of habitability. This requires conditions more excessive than no air conditioning. Such as no water, sewage backing up into drains, no electric service. What you can do, if ac is provided for in the lease, is either get the ac fixed yourself and sue her for the cost, or sue her for failing to provide ac. If you educt rent in these circumstances, she can probably sue you for the rent and have you evicted. I'm sure that a lawyer would charge you a minimal amount to write a strong letter to her, but she probably won't renew the lease.