In design view, click the pdfCatID select list, then clic kthe dynamic button in the property inspector, In the Select value Equal to section, click the lightning bolt and select the pdfID column from the WADApdfarticles recordset.
On you body tag, you have the Poiulate list from array behavior set correctly, but you dhave set the Set Selected In List behavior to use the pdfCatID list instead of the pdfSubCatID list, edit that so it is using the child list.