Inkscape can import PDFs and then save as "LaTeX with PSTricks macros" which essentially works by embedding PostScript into the LaTeX source. It's more trouble than its worth, and the resulting Latex source has to be preprocessed before it can be output as a PDF again.
Anyway, even with some hypothetical PDF to LaTeX compiler, at best you'd get something where the position of and size each character or word is separately specified -- the opposite of what you want, which I'm guessing is for a denominator to be one half of a fraction, rather than some number below a horizontal line.
