My xpdf is in /usr/bin and the permissions in the mimeTypes.rdf are the same as yours.
Just to be sure I made a symlinc in /usr/X11R6/bin which changed nothing.
In Edit Helper Applications I have:
MIME Type: application/pdf
Open it with: /usr/bin/xpdf
Then it says under file type details:
When encountered: Open these files with xpdf
Clicking OK I get Warning
Seamonkey can handle this file type internally. For such types a helper application will only be invoked if the server requests external handling.
Cancel Proceed anyway
On the Tab bar window it says: file.pdf (application/pdf Object)
If I make gv non-executable I get a blank page only on seamonkey and at the bottom it says Running Helper ...
BTW firefox and opera open .pdf files with xpdf on their own and the site in question is: http://www.vistamar.cl/descargas/ingles/etiquetas/Vistamar_Me.pdf
In firefox and opera xpdf is opened externally whereas in seamonkey it is, would I call it embedded within the browser in a new page. And I just checked, gv can open the file properly when invoked outside seamonkey.