Double-clicking on an attachment just seems to do nothing. On regular Evolution, I am able to preview such PDF before I send off the email.
I’m seeing this as well with Evolution from flathub. It’s probably something related to Flatpak and how Evolution calls on another program to open images.
As it works from received messages (related issue), I’m guessing this was fixed only for those and not while composing.
I’ve opened an upstream issue @ https://gitlab.gnome.org/GNOME/evolution/-/issues/1196 as they’re handling some flatpak-related problems there too.