PDF readers are a far more diverse bunch than browsers. It's much less likely someone has a simultaneous break on Adobe Reader/Foxit/okular/evince/xpdf/mupdf/et

PDF readers are a far more diverse bunch than browsers. It's much less likely someone has a simultaneous break on Adobe Reader/Foxit/okular/evince/xpdf/mupdf/etc. than on merely two browser engines (Chrome and Firefox).