As PennDOT announced recently, you no longer need these registration stickers on your plates. So you can just pay online and drive right after that. You no longer need to wait for mail, peel that card and stick it onto your license plates.
Following inforamtion is dirrectly taken from the official website http://dmv.pa.gov
PennDOT's answers on main questions:
Why is PennDOT going to stop issuing registration stickers?
When registration stickers are eliminated, customers can enjoy the convenience of paying their registration fee online from home or a mobile device, printing and signing a permanent registration credential and going about their business, without waiting for delivery by mail
Do I still have to register my vehicle?
Yes; Act 89 merely eliminated the need to place a registration sticker on your license plate effective December 31, 2016.
How will law enforcement know my car is registered?
Law enforcement agencies have real-time electronic access to PennDOT’s database to verify a vehicle’s registration. You are still required to maintain a valid and current registration card and present it to law enforcement when asked. Also, the valid and current registration card must still be presented at safety inspections.