To Integrate paypal in iOS App use the steps below:

1- Install Pod for paypal

platform :ios, '6.0'
pod 'PayPal-iOS-SDK'

2- #import <PayPalMobile.h> to your viewController.h and also set the delegate in .h @interface myPaymentPaypalViewController : UIViewController<PayPalPaymentDelegate>

3- Create an object for PayPalConfiguration *payPalConfiguration in ViewController.h file
Here is the code for paypal Cofiguration
 

- (void)viewDidLoad {
    [super viewDidLoad];

PayPalMobile initializeWithClientIdsForEnvironments:@{PayPalEnvironmentProduction : @"PRODUCTION_CLIENT_ID",
                                                           PayPalEnvironmentSandbox : @"SANDBOXCLIENTID"}];
    [PayPalMobile preconnectWithEnvironment:PayPalEnvironmentProduction];

                          //OR

[PayPalMobile preconnectWithEnvironment:PayPalEnvironmentSandbox];

    _payPalConfiguration = [[PayPalConfiguration alloc] init];  

// For example, if you wish to accept PayPal but not payment card payments, then add:   

_payPalConfiguration.acceptCreditCards = NO;

    // Or if you wish to have the user choose a Shipping Address from those already
    // associated with the user's PayPal account, then add:
    _payPalConfiguration.payPalShippingAddressOption = PayPalShippingAddressOptionPayPal;
}

- (IBAction)pay {
    
    // Create a PayPalPayment
    PayPalPayment *payment = [[PayPalPayment alloc] init];
    
//    // Amount, currency, and description
    payment.amount = [[NSDecimalNumber alloc] initWithString:@"1.0"];
    payment.currencyCode = @"USD";
    payment.shortDescription = @"By Paying $1 You will Get 10 life to play the game";

    payment.intent = PayPalPaymentIntentAuthorize;

    payment.shippingAddress = nil;

    if (!payment.processable) {
        NSLog(@"Request can not be processed!");
        return;
    }
    PayPalPaymentViewController *paymentViewController;
    paymentViewController = [[PayPalPaymentViewController alloc] initWithPayment:payment
                                                                   configuration:self.payPalConfiguration
                                                                        delegate:self];
    
    // Present the PayPalPaymentViewController.
    [self presentViewController:paymentViewController animated:YES completion:nil];
}

- (void)payPalPaymentViewController:(PayPalPaymentViewController *)paymentViewController
                 didCompletePayment:(PayPalPayment *)completedPayment {
    // Payment was processed successfully; send to server for verification and fulfillment.
    
    // Dismiss the PayPalPaymentViewController.
    [self dismissViewControllerAnimated:YES completion:^{
       
NSLog("Payment done");

    }];
}