What happens when you try and open it? Do you get an error message? If so, what does it say? What is the file name and extension?
An older PDF version should open fine but we need more details (as many as you can give) to know what is happening.
when I try to open a pdf document a new window appears like if it really open but nothing happens, the page just keeping gray.
So it's actually opening within Reader and not within the browser winow?
If so, it could be a scanned image. Try going to Edit>Preferences>Page Display>Page content and information and be sure that "Show large images" is checked.
OMG!!! problem solved I had this problem a month ago and until now I couldn't fix it. Thank you so much graffiti for helping me.