[lucida] Lucida OT equality symbol looks faint in Adobe Acrobat

[Kai von Fintel]

A pdf produced via XeLaTeX from the following minimal tex file (using 
the lucida-otf support files from CTAN) displays fine in many pdf 
viewers but in Adobe Acrobat the plain = and its italic version show up 
faintly or in outline. I attach a screenshot of how it looks in Acrobat.

This is a mystery to me. Does anybody have an idea of what might be 
happening?

— Kai.

```
\documentclass{minimal}
\usepackage{lucida-otf}
\begin{document}
\begin{tabular}{ll}
plain:  & =\\
textbf: & \textbf{=}\\
textit: & \textit{=}\\
math:   & $=$
\end{tabular}
\end{document}
```
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