Display a PDF (or image (sequences) in other formats) in the background (instead of solid color), and render on top. Enable jumping to specific PDF pages (or images in the sequence) using Scripting.
I would like to split the requested features. Current master version of OpenFlipper uses QOpenGLWidget which has an alpha channel and uses the compositor. Adding the ability to set the background of the OpenGL Widget to a transparent color is a minor change and would allow you to view whatever you like using your System pdf viewer, webbrowser etc.
You could try navigating the pdf / webrrowser whatsoever using keybinds of your OS. If you still require more control specifically for pdf files, we can discuss embedding pdf viewer capabilities to OpenFlipper by means of a plugin.
I will keep you posted regarding the transparency support.