Bug introduced in 10.0.1 or 10.0.2 and fixed in 10.1.0
Take this example plot:
plot = Plot[x, {x, -1, 1}, Frame -> True, FrameLabel -> {"x", "\!\(\*OverscriptBox[\(x\), \(_\)]\)"}]
When I export it to PDF, the overbar in $\bar x$ goes missing.
Export["~/Desktop/test.pdf", plot]
This is with Mathematica version 10.0.2 on OS X and looks like a bug to me. Is there a workaround? All I need for now is a workaround.