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No, and yes. It's proprietary and Windows/Android/iOS-only.

That's false, you can download documentation for the OneNote file format here

https://msdn.microsoft.com/en-us/library/dd924743(v=office.1...

it is only about 5% of the size of the Adobe PDF specification and I am sure there are corner cases, but it is not too bad.

I admittedly did not know that they published a specification . It's still a proprietary file format though. Proprietary does not mean "binary and undocumented", it means that it's not an open standard.

Toss Mac in there as well:

https://itunes.apple.com/us/app/microsoft-onenote/id78480155...

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