In case you
save a pdf files in document Library and other user want to open to open,then
SharePoint has not facility to open it in browser.When you click the link it
asks to save in hard drive.
By using PowerShell
command we can easily add the PDF MIME type to the web application's AllowedInlineDownloadMimeTypes property
$webApp = Get-SPWebApplication("http://abhay-pc:4444")
you run the above code and open the pdf,its open in the existing browser