To activate Apple push notifications for your Yumpu App navigate to the respective Appkiosk entry in your Yumpu Useraccount by clicking on the "My Appkiosk" Tab

Click on the "Tasks"  Button and select the "Settings" Tab
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Navigate to the "APNS" Tab and click on the "Activate APNS" Button
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You will be prompted to add the "APNS certificate" as well as the "APNS Private key" 

 The information for the "APNS certificate" as well as the "APNS Private Key" can be aquired by opening the ".pem" file with a text editor. Please refer to this guide on how to create a ".pem" file on your Mac.
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Copy the text presented in the ".pem" file into the respective fields (example screenshots shows a cropped text in both fields):
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Finish by saving and closing the window.