According to MSDN Knowledge Base article Q109397, DDE does not support long file names, so DOS alias names needs to be used for directory and file names longer than eight characters (i.e. C:\mydocu~1\file.mdb).
Different Microsoft Office applications differ in their support for the use of long file names when used as DDE Clients. For instance, Microsoft Word does not appear to support the use of DDE data item names which exceed 8 characters, while Microsoft Excel however, accepts long data item names if enclosed within single quotes. See Posting select CitectSCADA data using DDE for an example.