I managed to open 2 pdf files in Gimp 2.6.11 but when edited and saved, there's no pdf file extension for it.
Have you tried to open other pdf files and what version of Gimp you're using? (to see the version go to Help ->About)
Get involved! (see UI Brainstorm!). Discuss or seek help from other Gimp users and developers. Be part of the Gimp mailing lists to create and receive daily topics. Gimp Users and Gimp Developer are good to try.