rjbull - thanks for the suggestions and the lateral thinking. Not sure I understand what to do with FILE_ID.DIZ, but it would seem to involve typing descriptions, which would be difficiult for large numbers of PDFs. I was already thinking of digging out my old copy of "Where Is It" which is a CD/Disk catalogue application, like Disk Explorer Pro, but a bit more sophisticated, I think. As for File Hound, that is really useful, especially when associated with Folders in Explorer (right-click folder and select "Hound"). THE PDF text-based preview is very helpful. However, real-time search of PDF folders on my NAS is a bit slow, (which is why a tag/database system like Benubird would be nice). Better than searching via Explorer though.
Thanks again - I am sure there is something out there - just hard to find...