PDshop has built in support for PayPal Payments Pro, also known as Website Payments Pro.   To use this service with PDshop, all you need to do is locate your API credentials and copy and paste them into PDshop's Admin & content manager.  Once you enable it in PDshop, you can start receiving & processing credit card payments right on your website.

In your PayPal account under "API access" you will find the API signature.  You simply copy and paste that along with the API User name and Password into PDshop.  You do not need to install any certificates or download anything from PayPal, you just copy and paste those into PDshop's PayPal settings.

On the PayPal website, the Signature can be found under My Account > Profile > My Selling Tools > API Access, then once on the API Access page click "View API Signature".