The user manuals for all Huygens products can be downloaded here.
If you have the Acrobat Reader Plugin installed for your browser, it is possible that your browser will not prompt you to download the file but rather shows it directly. In case you are running Microsoft Explorer sometimes only grey pages are displayed. The workaround is to download the pdf file and view it with Acrobat Reader directly form your harddisk.
Proceed as follows: move the mouse over the link, hold down the SHIFT key and press the left mouse button. Alternatively, move the mouse over the link, press the right mouse button and select 'Save link as' from the popup menu. Double click the icon representing the downloaded file to launch Acrobat Reader.
You can find more online documentation at HuygensDocumentation.