\documentclass{article}

\usepackage{amsmath}

\usepackage{unicode-math}

\usepackage{amsfonts}

\newcommand{\boxright}{\ensuremath{%

  \mathrel{\Box\kern-1.5pt\raise1pt\hbox{$\mathord{\rightarrow}$}}}}

\begin{document}

$A\boxright B$

\end{document}

    but it doesn't work, I thought these packages are pandoc can handle  (https://pandoc.org/MANUAL.html). 

Adding a package won't help in this case. The texmath library
doesn't understand \boxright.

However, you could write your own macro to redefine
\boxright to something pandoc can handle, and include
this macro in your tex document.

吴小安 <nant...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:

> Hi, I try to convert a tex  document  to word, everything is fine, but the
> $boxright$ can not convert ([WARNING] Could not convert TeX math 'A
> \boxright C', rendering as TeX: A \boxright C )
>
>  I don't know how to add  "\usepackage{pifont}" to the pandoc, could you
> tell me how can I do it ? Thank you very much !
>
>                    
>
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