I’m completely unable to add “files” to notes, specifically PDF ‘s . How can I do this with the iOS apps?

Hello,  climate-control.

In QuickBooks Online, you have the option to add attachments in mobile app , some transaction aren't automatically link . I recommend to do it through using browser. You can consider looking this article: Compare mobile app features.

Also adding this article for a further overview of uploading documents in QuickBooks: Attachments in QuickBooks Online.

Feel free to visit this article to find answers to your other QBO mobile questions: QuickBooks Online Mobile app FAQ

Let me know in the comments below how it goes. If you have other concerns about managing your QBO app, I'll be right here to help. Keep safe always.