I discovered that defaults settings are stored in /var/cache/gio-2.0/defaults.list

with the command for Gnome of
gnomesu gedit /var/cache/gio-2.0/defaults.list
that opened gedit and in the line that says application/pdf= I inserted
[QUOTE]application/pdf=/opt/Adobe/Reader8/Resource/Support/AdobeReader.desktop[/QUOTE; I saved the file, and exited;

I subsequently discovered that I could have saved any pdf file that I had downloaded; right-clicked on it; and in "properties" I could have selected "open with" and set the Adobe preference from there; so two ways that work;