SVG

SVG is a graphical format which is scalable. It means that when you zoom an image, you don't loose quality. So this format gives good printing result.

You can enable SVG generation by using -tsvg flag with the command line.

You can also use format="svg" in the ant task definition. 

  <target name="main">
    <plantuml dir="./src" format="svg" />
  </target>
You can also generate SVG directly from Java.

Specific SkinParameter

You can change the target value in the generated SVG with the svgLinkTarget setting. 
@startuml
skinparam svgLinkTarget _parent
start
:[[http://plantuml.com]];
stop
@enduml
There is also a setting to specify a color change when the mouse pointer goes hover some links: 
@startuml
skinparam pathHoverColor green
class Foo2 [[http://www.yahoo.com/Foo2]] {
  +double[] x
  +double y
}
Foo2 --> Foo3
@enduml