Downloaded a flatpak file from Github. What now?

I went to and downloaded a flatpak file, thinking I could use it to install openboard view on my linux distro (21.1). I have no idea what to do next. Any help will be much appreciated.

Do you mean evince?

In Gnome Shell or KDE Plasma Desktop, if you click on it, it will automatically open the Software Center to install the package.

If you use another desktop environment that doesn’t have this file association, you can install from the terminal using:

flatpak install [path_to_file]

Considering that the response of @cho2 is correct and the file is in the Downloads folder:

flatpak install ~/Downloads/evince-43.alpha12707f9.flatpak

Thank you, but now I get this error:
‘error: The application org.gnome.Evince/x86_64/master requires the runtime org.gnome.Platform/x86_64/master which was not found’
Please forgive my ignorance, but how do I do this? I tried doing the apt install but it said the package could not be found.

you should just install evince from flathub and ignore that package.

The GNOME “master” runtime which is not stable, isn’t available on flathub.