If you are running the 64 bit version of PDF Studio 8.3.1 or earlier (pre Java 1.6.0_37 builds) on Ubuntu with the GTK or Nimbus look and feel, you might see extra space on the toolbars in PDF Studio.
There are two simple solutions to this:
1. Switch to the Metal or Substance look and feel
2. Upgrade to the latest version of PDF Studio, 8.3.2, Linux installer