I was using following VBscript code successfully in the past:
set MyPDFInfos = CreateObject(“AlambicEdit.PdfInfos”)
set MyPDF = Watch.GetPDFEditObject
MyPDF.Open Watch.GetJobFilename, False
MyPDFInfos.Keywords = “KeyWord1;Keyword2”
MyPDF.SetInfos MyPDFInfos
MyPDF.Save False
MyPDF.Close