There are several ways how you can get documents into the app. The most common ones are the following:
- iCloud Drive: You should have an iCloud Drive folder on your Mac. In there is a PDF Viewer folder where you can just simply drag and drop your files into and they will appear in the app's iCloud Drive folder.
- Mail: You can just simply send yourself a mail with the document as an attachment. On your mail app you can then open the document, tap on the share button and choose "Open with PDF Viewer" which will automatically open the document in the app.
- AirDrop: Use AirDrop to share files across Apple devices.
- Share from apps: If you have the document in another app, there's a good chance that the app has the ability to share a document.
- iTunes File Sharing: The file transfer is best described in Apple's official guide.
- Wi-Fi Drive: You can learn more about that in this article.