From Central Administration > Application Management > Application Security > Authentication Providers, select a Web application and the zone you want to modify. This is usually default.
In the middle of the page, check Enable Anonymous Access and choose Save
All site collections in that Web application can now have anonymous access enabled.
Go to a site collection in the Web application you just enabled anonymous access for
From Site Actions > Site Settings, open Advanced Permissions
From the Settings drop-down menu, select Anonymous Access
For this example, enable anonymous access for Lists and Libraries and click OK
Browse to any document library in this site collection
From the Settings drop-down menu, select Document Library Settings
In the Permissions and Management column, select Permissions for this document library
From the Actions menu, select Edit Permissions to break inheritance
From the newly appeared Settings drop-down menu, select Anonymous Access
Check View Items and click OK.