# Ligatures in figures and pdftex

In the following example, the 'ff' ligature in the figure labels (in 'differential') disappears when pdflatex is used, but the fault does not occur if latex-dvips-ps2pdf is used instead. Both lualatex and xelatex produce the correct results, using the pdf version of the figure. Is there a problem with the figure, or is this a bug in pdftex?

\documentclass{article}
\usepackage{graphicx}
\begin{document}
\includegraphics[width=0.9\textwidth]{Cluster2}
\end{document}
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