uPDF is an editor tool to write and paint to an existing pdf file. uPdf can insert pages of another pdf, insert new pages, remove and extract pages, rotate pages, paste a text or an image in the existing pdf, and much more.
You can install it in Ubuntu using ppa. For the moment, as I checked on https://launchpad.net/~atareao/+archive/updf, the uPDF ppa supports only Ubuntu 12.10 and 12.04. To install it, open a terminal and run next commands:
sudo add-apt-repository ppa:atareao/updf
sudo apt-get update
sudo apt-get install gir1.2-rsvg-2.0 updf
If you use KDE you may also need to install the following dependency:
sudo apt-get install-0.1 gir1.2-appindicator3
More info: Install uPDF (effective pdf editor) in Ubuntu 12.04 /12.10
Another tool for editing pdf files and that can be found in Ubuntu Software Center is Master PDF Editor - a complete solution for view, print and edit PDF and XPS files.