If you use Microsoft OneNote with very large notebooks that were originally created on a PC or Mac, opening and syncing such notebooks on your iPhone or iPad for the very first time can cause some performance and stability issues.
If you are experiencing such issues while connecting to large notebooks, turn off the automatic syncing of note attachments by following the steps outlined below, and then try syncing your large notebooks again.
Note: This documentation applies to Microsoft OneNote for iPhone and iPad. It is recommended that you keep your devices updated with the newest version of Apple iOS. For more information, see Update the iOS software on your iPhone, iPad, or iPod touch.
iPhone & iPad
To turn off automatic syncing of note attachments in OneNote on your iPhone or iPad, do the following:
In OneNote, tap the ≡ menu, and then tap the Settings icon.
Turn off the Auto Sync Attachments option.