Complete editing of pdf documents is made possible with PDFedit. You can change either raw pdf objects (for advanced users) or use predefined gui functions. Functions can be easily added as everything is based on a scripts.
Scripting is used to a great extent in editor and almost anything can be scripted, it is possible to create own scripts or plugins.
PDFedit can be installed in Ubuntu Software Center directly, by searching PDFedit.
website: http://pdfedit.cz/en/index.html
