What makes something a publication is the act of publishing, not the format that it takes.¹ Copyright is implicitly granted at publication² although registration is required in order to sue for infringement.³

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1. Within some limitations: certain types of creative works, most notably typefaces, are excluded from copyright law, although it was determined that digital font files that describe the outlines of the characters are programs and thus eligible for copyright. Bitmap font files on the other hand, as an expression of a typeface design are not eligible for copyright.

2. Although works created by federal employees as part of their job are explicitly excluded from copyright protection.

3. Note though, that the timing of the registration impacts what you can sue for. If registration takes place after the infringement you can only sue for actual damages, but if it takes place before the infringement you can sue for punitive damages.⁴

4. I should add the obligatory disclaimers that all of the above only describes US copyright law and also I’m not a lawyer (although I did used to watch Law and Order a lot) so everything in this comment could quite likely be completely wrong.