It isn't. All primary SEC filings must be in SEC-modified HTML. pdf is only permitted as secondary copies of the primary .HTML documents or as supplemental filings.
Now that the SEC has phased out text and ASCII filings and are .HTML only, it is harder for an amateur to file since all filings have to be in SEC specific HTML coding, which is much different from regular .HTML. I suppose a very good coder could do it by hand, but it typically requires specific (and expensive) software to convert it into the proper format.
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