Odd charecters

I have an issue where my font is not showing the scientific formulae within the pdf. The pdf-preview does correctly show the font as it should be rendered:

This is the correct format:

β-gal

and this is what is rendered in the pdf:

?-gal

It cannot find the font I assume although I believe it to be the symbol font and the dompdf font folder has this installed as default.

Can someone suggest a fix?

thanks

Jon

https://ww.wp.xz.cn/plugins/wp-pdf-templates/