Assuming you're using Ubuntu 11.10, running the following in a terminal should do the trick:
gsettings set org.gnome.desktop.thumbnailers disable "['application/pdf']"
Alternatively, you can do this in the GUI by installing the dconf-tools
package and running dconf-editor
. In the tree on the left, navigate org → gnome → desktop → thumbnailers
, then edit the disable
option in the right pane as above.
I'm not sure what specific vulnerabilities you are trying to avoid (if they existed, I'd expect you could avoid them by installing the corresponding Ubuntu security updates), but the above should prevent generation of new PDF thumbnails.