The PayPal iOS SDK provides a software library that makes it easy for iOS developers to accept both credit cards and PayPal directly within native mobile apps.
For usage and integration instructions, see the PayPal iOS SDK Integration Guide.
PayPal is replacing the Mobile Payments Libraries (MPL) with the iOS/Android SDK. The PayPal iOS/Android SDK is currently available in the US, with more country support coming soon. US developers should upgrade now for more features and a better mobile experience. Non-US developers, you can continue to use MPL for the time being. Issues related to MPL should be filed in the sdk-packages repo, not here.
For developers with existing Express Checkout integrations or who want additional features such as authorization and capture, you may wish to use Mobile Express Checkout in a webview.