When Adobe first released the PDF standard it was actually a proprietary standard and it was like that until recently (around 2008) that they realized to make it an open standard. I think it also must have helped the open-source developers to develop PDF editors with ease too (you know, developing an app for a "closed-source" standard can certainly bring some heat towards you :D) .
Anyhow, for all you guys who're searching for a PDF editor for Ubuntu, after a bit of a search, I came up with a one called "PDFEdit" which I then realized is something that's quite popular too.
And unlike with many other "text containers" in general, PDF is a single, platform Independent container that holds all the necessary information about the document such as Font, graphics (GUI), basic layout, images, encodings, etc which are called "objects" to render the file itself.
|As mentioned before, this is not just a PDF "text editor" but rather a complete and powerful PDF object manipulator, thus won't suit all the users...|
*. As said before, change operators and their parameters.
*. Various tools such as : Text selection, flattern, PDF to XML conversion.
*. Enable/disable property window.
*. Command-line support.
*. Change font, color, zoom in/zoom out, rotate, draw rectangles, lines, etc.
*. Save PDF pages as images.
*. Extract text from the page... are just a few to mention.
You can install PDFedit editor in Ubuntu 11.04 Natty Narwhal, 10.10 and 10.04 by using the below command in your terminal window.
sudo apt-get install pdfedit
As said before, I'm not the expert on these subjects and I'm pretty sure there are many other open-source PDF editors out there... if you know some, then you're more than welcome to tell me about 'em :).