Good day,
Thanks for your post in Microsoft Community.
I understand from your description that you can't attach external files to OneNote on on iOS or iPad OS.
We tested your situation by create a word file in Word app, and attach it to OneNote page and can successfully open the file in Word App.
Could you please tell me what kind of file do you want to attach in OneNote and where is the location of the files?
Feel free to post back if you need further assistance.
Best Regards,
Silvia | Microsoft Community Support Specialist
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