You’ll be pushing the boundaries, but it should be possible.
720 minutes (12 hrs) of audio will fit on a single DVD-5 if you encode it at 96 kbps (4,147,200,000 bytes). Audio quality will be fairly poor, which might not be so much of an issue if they are all lectures/lessons. This leaves you about 552,800,000 bytes free.
You would want to put each audio clip into its own timeline, and create a still frame for the video if you want any video information. Each still will take up at most a few hundred kilobytes. Menus will be about 50 KB each.
Provided your PDF files are not too large, it should be possible. Another word of advice: SaveAs the project often.