This may have been asked before but I can’t seem to find it.
With Apple notes, once you’ve finished with a note, you can select, share, save to files, and then choose a location in the files app to save the note, this then sends copies of text, photos, pdf, or whatever is in the note to a folder, which is great because you have a natural archive, let’s say, under a customers folder you created, and the original note can be deleted, or in agenda it could be archived.
I can’t find a way to share from agenda to the files app in this specific way.
You can export as an Agenda File or a Rich Text with Attachments file. Both will keep the attachments, though the Agenda File is most complete (it is an exact copy).
So use the Export option to use one of those. On the Mac there is File > Export and the Share menu that you can use. On iOS, locate the Share option and use that to add to Files.