Hi Paul, thanks for your reply.

I tried as per your suggestion, here is what amended:
{ MERGEFIELD DocNumber \# "'DO-'0000" }

however, the result was still not as desired.

By the way, below the original, just in case this will help to indicate what goes wrong.
{ MERGEFIELD DocNumber \* MERGEFORMAT }

Hope you can help again.
Thanks.

Boon Lai