Handling files securely in Xamarin Forms

I am implementing a cross platform app which will pull PDF documents from a Web Service. These PDFs need to be completely inaccessible from outside and can only be viewed when authenticated in app. These PDFs can be stored for offline viewing.
I am currently writing the byte[] of the file to an excrypted SQLite database. However, my concern is loading a whole Document into memory at once. (Note: due to the nature of these Documents, they will never exceed 100MB). Is there a recommended way to securely store Documents, or a way to properly handle the transfer of files from HTTP to SQLite?