Do I need to use PayPal SDKs to call PayPal APIs?
No, you don't need to use PayPal SDKs to call PayPal APIs. However, the PayPal SDKs ease the task of calling PayPal Web Service APIs. They contain libraries that shield the user from the intricacies of the SOAP protocol, and offer a number of other useful features.
You can invoke PayPal Web Service APIs from any system that supports SOAP communication with client-side certificates. The SDKs are very helpful for this purpose, but they're not required for using the PayPal Web Service APIs.