If i have on my site a link to a pdf that contain part of the content of the site will be considered duplicate content?
You can make the link to your pdf out of reach of SEs. This way there will be no duplicate content trigger for Google.
You can optimize your PDF file by so that it will not be considered as duplicate content by SE..... try this link: http://help.adobe.com/en_US/Acrobat/8.0/Standard/WS58a04a822e3e50102bd615109794195ff-7c67.html
other question: is the content of a pdf considered with the same weight (keyword wise) of html content?