Hi PastlTGuy,
Thank you for your message in this forum.
We understand that you want to export or download notebooks from OneNote for Windows 10, however, there is no out of box way to achieve that in OneNote for Windows 10.
We are sorry as it cannot fully meet your requirement.
If you want to share the notebook with a partner from another company, in addition to using OneNote 2016 to export notebooks, for a workaround, we’d suggest you can use Share feature to share your notebook to the others.
You can go to OneNote for Windows 10 > click Share, as shown below:

Then you can enter the email address of that partner, and click send. That partner can go to his email and access the notebook you shared.
At the same time, if your notebooks stored in SharePoint Online, you can select the notebook you want to share > click Share in the Navigation Bar > then you can select the Specific people and type the email address of that partner.
For more details, you can refer to this article: Share SharePoint files or folders.
We understand the situation and you make a great point.
To address your concern about the situation, we’d suggest you kindly vote this UserVoice:
Export for OneNote pages, sections, section groups and notebooks. This is the best platform to let us hear from you and make our products and services better for you and others.
We appreciate your understanding!
Best Regards,
Sukie