Hi Graeme,
I just tried this in Power InRoads V8i SS2 version 8.11.07.615 and was not able to reproduce any problem in relation to re-opening DGNs that have PDF attachments. The 8.11.07.615 version is available for download from SELECT Services and there is also a SELECT Series 3 release available (8.11.09.493) which includes the OpenRoads technology ( www.bentley.com/.../OpenRoads ).
Regards
Paul