No, PayPal is not offered in all casinos. PayPal is a relatively new solution, and all casinos are yet to absorb the technology and make it part of their payment processing system. PayPal is also not available in all countries.
It is, however, important to note that most of the reputable casinos in the world accept PayPal, so there is no shortage of casinos which support this method. Always look out for the PayPal logo at the bottom of casino websites, or read casino terms and conditions to find out if they accept PayPal or not.