Bug: ArcMap does not support overprinting or knockouts for printing or exporting
ArcGIS supports color overprint definition (sometimes called overprinting or knockout) in file export through the Esri Production Mapping and PLTS extensions. In PLTS versions 9.3 and 9.3.1, and in Production Mapping version 10, overprint definition is supported for EPS format. In version 10.1, Production Mapping supports overprint definition for PDF format. ArcGIS does not support overprint definition for print and export unless the Production Mapping extension is installed.
This is a known limitation.
Esri is aware of the enhancement request to support overprint definition in the core print and export functions. These are being considered for a future release.