Nothing special, except you have to declare the font in the CSS, and use the internal name of the font (in the case of Montserrat, it looks like it is “Montserrat Bold” with no hyphen)
For example the following template gives the PDF attached.
Thanks for the update; this is not the same as being able to use to get the bold font or for italics; this looks like an explicit assignment of the font to a class. Is it possible to assign a font family for it to pick up the correct custom font as it would if using something like a serif?