Calander app - how to add scans of letters

I have a hospital appointment in a few weeks.

I noted the appointment in my calendar and was surprised that I experienced difficulty in adding a scan of the appointment letter to the calendar entry. I cannot understand why this would not simply be straightforward and intuitive. Nor can I understand why my iPhone would start searching iCloud when I attempt to add an attachment, not why it would display an entirely random assortment of files. If I have just scanned the letter in notes, what good does it do it search in iCloud?

However, my question is - can I add a scan of my hospital appointment letter to the relevant entry in the calendar app? Yes or no? If yes, how do I do this?

Thanks for any and all help.

If this is not something that most iPhone users routinely want to do, I am honestly astonished by that.