# black background when including pdf with transparent background as image

I have a pdf image with transparent background that I'll include by using graphicx (\includegraphics). Everything is fine when using pdfLaTeX to build my pdf, but when I switch to XeTeX the image will suddenly have black background instead of being transparent. I have tried to reproduce the problem, and quite strangely some pdf images do not exhibit this problem while some do.

My XeTeX version is 3.1415926-2.4-0.9998, which I believe is the newest. And here's a sample pdf image if any one wishes to test: https://docs.google.com/file/d/0BxDcKKtrknW_MlZMV2lKOE5oMXc/edit

Minimal code:

\documentclass[10pt]{article}
\usepackage{graphicx}
\begin{document}
\begin{figure}[h]
\centering \includegraphics[scale=0.48]{MapReduce_Pipeline.pdf}
\caption{MapReduce Pipeline.}
\label{fig:MapReduce_Pipeline}
\end{figure}
\end{document}
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