Export ggplot digits containing special character in R?

I have created a figure using ggplot in R and I want to save it in pdf format. My problem is that my figure when exported (using ggsave or pdf function) in pdf does not contain a ≥ 8 character in the legend. Instead, I see ... 8 .
Also, I get a warning message:
In grid.Call(L_textBounds, as.graphicsAnnot(x$label), ... : conversion failure on '≥ 8' in 'mbcsToSbcs': dot substituted for <e2>

Does anyone know a way to fix this problem?
Thanks to