This article explains what an ONEPKG file is, how to open it on your computer, and how to convert a page, section, or notebook to another format.
OneNote – How to Open .ONEPKG Files in Windows (Easy!)
A file with the ONEPKG file extension is a OneNote Package file. It is an archive of multiple OneNote documents (.ONE), produced in OneNote by backing up a notebook full of pages.
Another file you'll find inside this file is a OneNote table of contents (.ONETOC2), which stores organizational information about how the different sections of the document are arranged.
ONEPKG files are opened by Microsoft's OneNote tool. You can download OneNote for free from Microsoft.