PDFedit is free and open source library for manipulating PDF documents, released under terms of GNU GPL version 2. It includes PDF manipulating library based on xpdf, GUI and set of command line tools.
Multiplatform library working on Unix systems, Windows32/64 and also Windows CE and others. You can use it to read, change and extract information from a PDF file. It is based on xpdf library.Graphical interface based on QT3.x which heavily depends on scripting; therefore, any user can modify the behaviour by scripts and plugins.

Ubuntu user can easily install PDFedit by this command:

sudo apt-get install pdfedit

Or,you can download the latest version from this link:http://pdfedit.petricek.net/en/download.html
and PDFedit can be launched from Applications->Graphics menu after installation.