Most "native" way of managing files

Hi again! I continue learning to use my iPhone. Now I'm into managing my files in a convenient way. I know the "natural" way for managing files on iOS is app-centered (so each document type is "saved" in the arena belonging to each app).

However, let's imagine you're preparing a presentation about a city you're visiting. Let's say you're visiting Venezia, and you're collecting material for your presentation. Such material is photos, notes, audio clips, and even some pdfs. If you were using a computer, you'll create a "Venezia" folder, and manage all your presentation stuff into such folder. However, on the iPhone... what would be the iOS-way of doing this? I suppose you don't have to go to the photo app for the photos, the note app for the notes, iBooks for the pdfs, and so on, do you?

I suppose there're third party apps for doing this, but I don't want to work in a way opposite to the iOS design.

What would be a good way, fully integrated in the iOS design, of managing files of different types that belong to the same subject?

Thanks!