Jeroen van Wilgenburg's blog

Moqups.com

Moqups.com – A free online tool with the ability to export to PDF. It’s possible to add links to element, but I’m didn’t manage to get it working with the PDF

Balsamiq

A paid tool, but well worth its money. The key feature is the PDF-export with working buttons! You can click a button in the PDF and it jumps to the right screen

Onotate

Onotate let’s you annotate things. Very useful tool, only drawback is that when you change files you have to upload everything again