To view a document, Adobe Acrobat Reader and it's plug-ins need to be installed properly on your computer.

Attention: PDF complete or PDF Architect are not supported

Ensure that you have indeed installed Adobe Acrobat Reader and the plug-ins needed.

Internet Explorer: Open PDF directly in Reader or Acrobat (quick fix)  

Firefox: View PDF files in Firefox or choose another viewer

Attention: Apple Mac users must also have Acrobat Reader installed.