Why would the method by which you transfer a item that has a copyright impact the copyright? You buy a new book at a new bookstore, a used book at a used bookstore, a used book at a garage sale, someone gives you a book, you find a book on the sidewalk, you steal a book from a store, you buy and download an ebook, you give an ebook to someone on a USB stick, you download an ebook via Bittorrent. The author's copyright - as well as the design copyright, and any book company trademarks - does not change in any of those scenarios.
Copyright around most of the world - read Berne Convention (Wikipedia) - says that copyright exists at the moment of creation of a work, i.e. a work that you say is "100% yours". This has nothing to do with the way the work may be transmitted or stolen or downloaded. A work in the public domain can still be copyrighted in terms of cover artwork and design, annotations, etc. Read Welcome to the Public Domain - Stanford Copyright and Fair Use Center
You can say someone "owns" a book in the sense that they might have paid for it or it is personal property and one could justifiably call it theft if someone took it from them, but "owning" the physical or electronic copy in any sense doesn't mean you own the copyright. Read the copyright notice on a book or ebook; you get a license to read it, not ownership of it. Read What's the difference between Copyright and Licensing? - Open Source Stack Exchange.
Sure, the TOS of a network can specify the ownership/licensing status of the files shared on such network. They will almost all explicitly say not to upload or share anything that will violate the copyright of that work. The TOS of a network could possibly say that anything that is uploaded is automatically licensed to them. A network could demand the reassignment of copyright upon upload, but that would have to be outlined in the TOS and is not simple. See Copyright Ownership and Transfers FAQs - Stanford Copyright and Fair Use Center.
