the need for a secure connection using SSL is determined by whether the user is entering sensitive information on your domain.
In the case PayPal Payments Pro, you have 2 services:
PayPal Express Checkout - This is a remote checkout method, the user enters payment information on the PayPal site, so no SSL certificate is needed.
PayPal Direct Payments - This is a Local Checkout Method, the user enters their credit card information on your site. In this method, you should have an SSL certificate to protect the users payment information.