You can find all PayPal SDKs on the PayPal Developer Portal.
How do I get started with PayPal APIs and SDKs?
How do I whitelist my server’s IP address so it can access PayPal APIs?
How do I integrate PayPal Payments Pro with CoreCommerce?
How do I integrate Payments Pro with Angell EYE for WooCommerce?
How do I integrate Checkout with Mal's E-commerce?