I had been looking for a good PDF editor in linux for a long time. In addition to a bunch of my favorite features, I was searching for a product that is stable, fast and is easy to use. Among the host of features, I was particular about highlighting, graphical annotations, comments and baloon notes insertion capabilities so I can very easily highlight, mark and add review comments to tons of research papers that I read and review regularly.
The first tool that I tried was "pdfedit" obviously because it was free. I commend the pdfedit development team for taking initiative and creating the pdfedit software. pdfedit offers many features but falls short in satisfying my top three criteria.
I then started searching for commercial solutions. After downloading trial versions and trying out many, I finally homed in on pdfstudio 6.0 version from Qoppa Software(
http://www.qoppa.com). The very first run of the trial version caught my attention. Easy to use and yet, it offered many powerful PDF editing features. It has remained stable and has won me so far. I learned that it is targetted to work on hosts of operating systems - linux, mac and windows.
Hopefully, others in the market will catch up soon to understand customer's needs and meet expectations as pdfstudio does(to quiete an extent). I am certainly looking forward to seeing that come from others(including pdfedit folks).