I was searching on the internet for the reason of this issue and fixing it so I found the following it worked with me fine and witout any needing to restart IIS
- First Looks like IIS is adding the http header X-Download-Options: noopen which forces the browser to download the file instead of opening it. This new header is used by IE 8 to protect from script injection attacks.
Those who have experience with SharePoint 2010 may have noticed that pdf files in SharePoint 2010 do not open in browser by default. SharePoint 2010 throws a prompt to save the pdf file.
This is by behavior in SharePoint 2010 and is very annoying especially when you have a requirement to show a pdf file in a Page Viewer Web Part, etc…
Luckily there is a workaround to fix this issue.
Open Central Admin > Application Management > Manage Web Applications.
Choose a web application and click “General Settings”.
Scroll towards the bottom of the page until you find the section “Browser File Handling”.
Change the option from “Strict” to “Permissive”.
Go back to the document library and click on the pdf file. It should open in the browser. (Perform an IISRESET if required.)
You may also have noticed that the icons for pdf files do not show up by default in SharePoint 2010.go to the following link to solve this problem
for further details go to