BUT:
Some links to pdf documents are not opened in 'XChange Viewer' plugin for Mozilla Firefox-23.0.1 (Win). Links to pdf files are opening the XChange Viewer in its own window:
What AndyK has posted is true, however in some cases the website simply sends instructions to your browser "download this file" - and even though you have the correct settings - the browser follows the instructions and offers you to save the file. Other sites simply say to the browser "this is a PDF file - handle it as you know" - and in that case the browser checks and sees that you have an add-on that can display that type of file directly in a window - and uses it instead of offering you the "save as" dialogue. So for the provided sample - we do not have much options as the website requests the browser to start a download process.
Yes most likely it would have been the same with any PDF Viewer installed - as the browser simply does not try to search for a PDF handler and instead directly offers you to select a save location for the file.
