Alaeddine
May I ask why you would want to view a PDF inside widget, from my own experience of trying to put a pdf help file into a widget it becomes to small to be useful. You are better off just opening up the pdf into a new browser, or use Roberts link widget to open it from an icon in the header:
http://gis.calhouncounty.org/FlexViewer3.0/index.html?config=config-link.xml
Regards
Anthony