The easiest way to open SharePoint-based OneNote notebooks on your iPad or iPhone is to use the free OneDrive app.
Do the following:
If you don’t already have it installed, download the OneDrive app from the App Store.
Sign into the OneDrive app with the same account that contains the notebook you want to open.
In the OneDrive app, navigate to the notebook you want to open, and then tap the notebook’s name.
Note: Your organization may require you to set up a VPN connection on your iPad or iPhone to access SharePoint notebooks when you’re not directly connected to your network. If you’re having trouble accessing a SharePoint site, or if you’re not sure if you need to use a VPN connection, please check with your administrator.
Sign in to an Office 365 SharePoint site from OneNote
To open a OneNote notebook on an Office 365 SharePoint site, do the following:
In OneNote, go to Settings, and then tap your account name.
Tap + Add a Service, and then tap SharePoint.
If prompted, enter your email address and password, and then tap Sign In.