I have been trying to do the same but experimenting instead with the greater flexibility provided by xelatex on the use of fonts. The following code produces a fine result but I am not sure if changing the sans-serif font won't mess with scanned elements.
exams2nops(questions, n = 2, dir = "nops_pdf", name = "test",
usepackage = c("unicode-math"),
header="\\setmainfont{TeX Gyre Pagella}\\setmathfont{TeX Gyre Pagella Math}\\setsansfont{Roboto}",
texengine="xelatex")
Can you comment, please?