There are several ways to get documents into the app. The most common ones are the following:
Cloud storage providers: You can open files directly from iCloud Drive, Dropbox, OneDrive, Google Drive, Box, or any other cloud storage provider that supports the standard iOS document browser.
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 Copy to 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. Check out the article on different ways you can import your documents from other apps into PDF Viewer.
iTunes File Sharing: The file transfer is best described in Apple's official guide.