I have a pdf generated from a word doc which uses wingding 2 symbol font for a checkbox. When displayed on a windows machine it displays properly, when displayed on apple/android device it displays as an S. Attaching the pdf, if you need the word doc let me know. Note that the pdf seems to include embedded fonts, but it includes one for wingdings 2 and one for wingdings2 (no space). Not sure if that is related or not. In the word doc the font was wingdings 2 (with space).
I posted this in the Spire.pdf forum by mistake, although I'm unsure which piece is responsible.