Use the registry keys below to combining jobs into a single PDF file. 
Each time PDF Writer appends a job, it decrements the counter until 0. You can set the CombinePDFs value to the number of PDFs you want to merge.
 
HKEY_CURRENT_USER\Software\Custom PDF Printer\CombinePDFs
(0 = Existing files are overwritten, 1 (>1) = appended job at the end of existing file.)
 
Note: This value is of type REG_DWORD..