You set it up as system default;
& U set in mimeTypes.rdf: handle with system default that is done via mtypes editor at the bottom of
Edit menu item.
OS part: Right click on a pdf file ( it can be a renamed fake txt
& at the bottom You chose Foxit or whatever application & confirm.
Alternatively most apps offer in their options to take responsibility for certain mimetypes / extensions. Chose that the apps of Your choise handle all they can do.
thx 4 reading my post