Originally, overprinting was not supported in eDocBuilder because it was not supported in the ABCPDF Library that eDocBuilder uses.
https://support.aleyant.com/kb/a294/is-overprinting-supported-for-the-master-pdf-in-edoc-builder.aspx
It appears that overprinting is now supported in ABCPDF, and it would be good to support this in eDocBuilder itself.
https://www.websupergoo.com/helppdfnet/default.htm?page=source%2f5-abcpdf%2fxrendering%2f2-properties%2foverprint.htm