You cannot use Eval with the DocumentContent column. DocumentContent holds the content of all editable regions on a page. Using Eval with the column's value renders the raw XML representation of the content.
To display an editable region using a transformation, use GetEditableValue(string controlId)
where controlId is the value of the Web part control ID property that you set for your editable region.
For example: <%# GetEditableValue("mainText") %>
Source: http://devnet.kentico.com/docs/7_0/cmshelp/index.html?newedit_transformation_methods.htm