So it’s not that it’s downloading as a PDF. That’s not a problem with LibFredo6 or Sketchucation.You’ve got Acrobat Reader set to open .rbz files. You need to fix Reader.
By the way, you don’t open .rbz files by double clicking on them. After you’ve downloaded the .rbz file, go to SketchUp’s Window>Extension Manager aand click Install Extension. Then navigate to where you saved the file, select it and click Open in the file explorer window.
Maybe this will help with the file association issue.