In many cases, when a user clicks on a document in a document library inside SharePoint 2010, the login dialog is displayed asking the user to authenticate. In most deployments this is not a required behaviour.

To modify this, go to SharePoint 2010 Central Administration->"Application Management"->"Manage Web Applications" then click on the web application you want, from the ribbon: select "General Settings" and go to the "Browser File Handling" property, change it to permissive.