One of the most commonly asked questions I get is ‘how do I get anonymous access to work’. Usually, a person has enabled anonymous access in Central Administration, but not in the site collection. Here, I will try to reduce the process to the bare minimum required to enable anonymous access for a single document library.

1. From Central Administration > Application Management > Application Security > Authentication Providers, select a Web application and the zone you want to modify. This is usually default.

2. In the middle of the page, check Enable Anonymous Access and choose Save

3. All site collections in that Web application can now have anonymous access enabled.

4. Go to a site collection in the Web application you just enabled anonymous access for

5. From Site Actions > Site Settings, open Advanced Permissions

6. From the Settings drop-down menu, select Anonymous Access

7. For this example, enable anonymous access for Lists and Libraries and click OK

8. Browse to any document library in this site collection

9. From the Settings drop-down menu, select Document Library Settings

10. In the Permissions and Management column, select Permissions for this document library

11. From the Actions menu, select Edit Permissions to break inheritance

12. From the newly appeared Settings drop-down menu, select Anonymous Access

13. Check View Items and click OK.

You should now be able to view items in this document library via anonymous access.

Source: Enable Anonymous Access in Windows SharePoint Services V3 and SharePoint Server 2007