I used to use pdftk to edit the PDF metadata of my PDFs before sending them to my Kindle.

Just googled for a GUI to use in Ubuntu and came across to this "made in France" application: gnome-pdf-tool.
Binaries not available for the Ubuntu 12.04 but works pretty well just out of the box.


The big advantage is that you don't have to export the metadata to a text file, edit them and then import them back.