\documentclass{scrartcl}
\usepackage{newpxtext,newpxmath}
\usepackage[a-2u]{pdfx}
\begin{document}
\pdfcompresslevel=0
\pdfobjcompresslevel=0
Some greek characters seem to miss unicode mappings: $\mu$  % <- verification fails; comment out to succeed
Works for others: $\Sigma$
\end{document}