Hello,
The easiest approach would be to use an url in the following format:
www.domain.com/getattachment/guid/filename.ext
The transformation should look the following way:
<a href="www.domain.com/getattachment/<%# Eval("customField")%>/fileName.ext" >
<%# Eval("DocumentName") %>
</a>
Some useful information can be found also in this
forum thread and our
documentation.
Best regards,
Boris Pocatko