μg becomes mg

I open below file from WHO in Acrobat Reader, copy '4 μg/kg' to MS Word and found it becomes mg, not μg. It is 1000 times higher than the original unit. If I copy the whole paragraph, I think I will not notice the error. The font of the μg is 'Symbol bs', but actually, wherever I copy to, like chrome, notepad..., it always becomes mg. If I use 'Find' to look for μg within the pdf, none can be located. On the other hand, if I find for mg, both μg and mg shows up.  

I would like to know is it a Acrobat problem or display/typing problem of the document?

https://www.who.int/water_sanitation_health/dwq/chemicals/seleniumsum.pdf