Pandoc is a powerful tool but it is not Obsidian-specific. You may have better luck looking for more general resources elsewhere on the web. Edit: Your request may sound simple, but I think there’s a lot of inherent complexity. How do you decide where to start and end? What about recognizing embedded content, especially notes embedded in other notes? What if you have an embed loop? Point is: this is not a trivial task, in my opinion.
Otherwise, Obsidian does not have an “Export all as PDF” feature at the moment. You may want to submit one to #feature-requests.