A variant of #1 or, usually, #2: most of the time when you "buy" a digital good, you're really renting it out under extremely limiting terms.
If the files are not protected by DRM, then there's no technical limitation on copying or redistributing the file, but according to your license agreement you're not permitted to do so.
In practice, no one is probably going to come after you for copying your music files or ebooks across your devices or sharing it with friends, but you don't own the file. Try mass distributing it or reselling it long enough and you'll attract someone's attention.