Export as multi-page pdf
It can already import an entire pdf document, which is great. The next logical step is to allow users to export the marked-up pdf as a new pdf document.
My Huion HS64 works great with the whiteboard, but direct editing on pdfs doesn't currently work well.
Siem Eikelenboom commented
The PDF converts to an image in MS Whiteboard, so any relation to the old PDF is gone. It seems to me you really want Bluebeam Revu with its Studio working like a shared whiteboard for a PDF.