thunderbird pdf association

[Skunk Worx]

I have Fedora 11 X86_64 and when I get a PDF in email via Thunderbird I 
double click the PDF attachment and it pops up with :

"somefile.pdf could not be opened, because the associated helper 
application does not exist. Change the association in your preferences."

Under T-bird preferences/attachments/view&edit I see PDF is set to 
"Document Viewer".

I have not yum updated for a week or so. Is there a way to get PDF files 
or "Document Viewer" working?

System Tools/File Browser does open a PDF file correctly in an 
application named "Document Viewer".

---
John