If fw Server-side printing results in "empty" or "zero bytes size" PDF:
=> Is the fw Server running on Apple OS X 10.11 "El Capitan"?
How to fix to make PDFWriter work again on Apple OS X 10.11 "El Capitan".
1) These instructions are based on using these instructions in regards to installing "PDFWriter":
Dropbox (farmerswife) > fw_documentation > Published > Reports_and_Print_Designer > PDF_Print_Support_for_Invoice_Email_and_User_Report.pdf
2) After installing the PDFWriter package go back and explore the contents of the installer package, then open up the .gz archive.
3) Copy the "PDFWriter.ppd" file from the archive (Archive/Library/Printers/Lisanet/PDFwriter/) to your machine to this path: /Library/Printers/PPDs/Contents/Resources.
4) Now go to the Printers dialog and add the printer, choosing the Lisanet driver as explained in the documentation.
IMPORTANT: If you add the printer in System Preferences before steps #1 and #2 PDFWriter will _NOT_ work.