A lot depends on whether the original PDF was properly tagged. If it wasn't then there's nothing for it but to got through the text searching for bad paragraph marks. If it was, then first export the PDF to html in Acrobat and then feed that to calibre.
Pablo is right, though, even a tagged PDF will need a reasonable amount of html editing to create a professional result, unless whoever created it was particularly scrupulous with the tags and carefully marked each style.
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