$ pandoc MWE2.md --mathml --filter=pandoc-crossref --filter=pandoc-citeproc -s -o MWE2.pdf  --pdf-engine=xelatex
$
$ pandoc MWE2.md --mathml --filter=pandoc-crossref --filter=pandoc-citeproc -s -o MWE2.docx

[WARNING] Could not convert TeX math ' \lim\limits_{r\to\infty} U_{X}^{\sigma} = -\mathlarger{\frac{1}{r}} \qquad(1)', rendering as TeX:

  igma} = -\mathlarger{\frac{1}{r}} \qquad

                     ^

  unexpected "{"

  expecting "%", "\\label", "\\nonumber" or whitespace