I had this problem where prints to our CUPS-PDF printer sometimes failed to be processed on the server. The job would disappear as though it has been printed but nothing else would happen. Printing from the same application to a Windows based PDF printer, and then printing the resulting PDF via Adobe Acrobat to the CUPS PDF printer would work fine. Printing the same PDF via Sumatra PDF to CUPS-PDF would also fail.
Further investigation revealed that the resulting print job files would differ. The jobs that fail looked like they contained a lot of binary data but the ones that succeeded looked like normal PDF files.
Then I discovered this entry in the Windows Event Viewer:
The document XXXX, owned by jason, failed to print on printer \\server\PDF. Try to print the document again, or restart the print spooler.
Data type: NT EMF 1.008. Size of the spool file in bytes: 2555904. Number of bytes printed: 0. Total number of pages in the document: 1. Number of pages printed: 0. Client computer: \\CLIENT. Win32 error code returned by the print processor: 0. The operation completed successfully.