The following minimal example does not compile:
\documentclass{mnras}
\usepackage{pdfcomment}
\begin{document}
\section{\pdfcomment[color={1.0 0 0}]{title}}
\end{document}
That is, pdflatex shows the following error:
! Package xkeyval Error: `COLOR' undefined in families `pdfcomment'.
See the xkeyval package documentation for explanation.
Considering that MNRAS makes all text inside of the \section uppercase, I assume this is the source of the problem, i.e. the section macro parses the text and makes it uppercase before it gets parsed by pdfcomment.
Is there any way to get around this without redefining \section (or some related macro)?
For reference, I'm using TeXLive 2020.20210202-3 on Debian Unstable.

\section. I don't know about the MNRAS class, but sometimes the section title is also used in the table of contents and the header of the resulting document. Can you say what you hope to achieve? – Willie Wong May 04 '21 at 19:11