Try right clicking on the PDF file, left click Properties and left click the
"Open With" tab and select from there. Let me know if that works in Ubuntu.
I'm using a Fedora 14 install on this machine.

Well, if I'm downloading through Firefox a dialog opens up asking what to do
with the file- wanting to know save or open with. The open with offers
Libre (default) or 'other' which brings up my home folder with a gazillion
alternatives including calculator and IsleRiot solitair! What I want to do
is change the default.
There is no pdf file to click on.

In that case, I would save it to the hard drive first and try it the way I
described. Personally, I'm not sure how to change the default any other way.

Yeah, that works but then I have to mess around to change it and probably
erase it whereas in 10.10 it just opened up in the reader and I could choose
to save it or just close it and it was gone. There must be a simple fix for
this, I just haven't found it yet

Is there something is your syetem preferences about "Preferred Applications"
that you can adjust?

Yes but it only concerns itself with Internet (browser & Mail), Multimedia
(Totem, banshee etc), System (Gnome or X) & Accessibility (Orca etc).

Firefox > File > Edit > Preferences > Applications
Scroll to PDF files and click on the arrow next to the application to
change to a new default.

A two part solution:

1) Find and on a "PDF document"
"Properties"
"Open With" tab
You should see "Document Viewer" and possibly other PDF viewers with
radio-buttons
Select your poison.

2) In Firefox
"Edit"
"Preferences"
"Applications" tab
In Applications there are two columns
On the Left is "Content Type" and on the Right is "Action"
Scroll down until you see "PDF" and it
on the down-arrow in the "Action" column associated with "PDF" (mine is
"Always ask")
Pick your poison. If you choose "Use Document Viewer (default)" this is the
viewer set-up in Step 1.

I haven't tried Step 2 so if we luck-out or strikeout let us know but it sounds
good. (I know from experience that Step 1 works.)

Your first part solution may have helped as Properties was indeed set to
Libre - now changed to Document Reader.

Part 2 is strange. I'm with you until 'always ask' but when opening 'other'
there is no 'document viewer' listed amongst the hundreds pf alternatives.

Maybe step one has fixed it, thanx a lot, if not

Here is Part 2 again with a few modifications

In Firefox
"Edit"
"Preferences"
In the Toolbar "Applications"
In Applications there are two columns. On the Left is "Content Type" and on
the Right is "Action"
In the "Content Type" scroll down until you see "PDF document" and
it
on the down-arrow
In the "Action" column associated with "PDF document" in the "Content Type"
column on the down-arrow (mine says "Always ask")
Pick your poison. If you choose "Use Document Viewer (default)" this is the
viewer set-up in Step 1.

I am guessing by download you mean using Firefox, I just looked
on my Firefox under edit/preferences/Appllications tab and for
PDF document (application/pdf) it says "Always ask" and has a
drop box for other selections, for save, viewer and something else.

I like the always ask so I can either save the file or view it to
see if I want to save it or not.