Q: When I download an aac file, where is it stored?

If you mean from the iTunes Store and are referring to your iPad,  you'll find it in the Music app.

Regards.

Are you talking about on an iPad? Or on a computer?

From where (or what app) are you downloading this file?

Then if it's actually downloaded rather than streamed, the file would be within the app you use to access the file, presumably the magazine app. The iPad doesn't have the same sort of file structure as does a normal computer, so you won't find the file separately.

Regards.