Browser seems like a bad way to do it. Probably the slowest app, most likely to be used for something else, and doesn’t do PDF…
Maybe try barwal as per Covpass - Vaccination status App? - #81 by amaretzek
Or you could bake your own .desktop file that invokes sailfish-office or jolla-gallery with the appropriate file as argument. You’d put that in /usr/share/applications/ or ~/.local/share/applications/
Not tested, but something like this probably (also see existing files for examples):
[Desktop Entry]
Type=Application
Name=Documents
Icon=<some icon name or path>
Exec=<what you want to run>
Maybe xdg-open /path/to/your/cert.pdf is the best Exec value, since that triggers the default open action and thus keeps to a single-instance of e.g. sailfish-office.