When opening a PDF file, an ERR_FREN_UNSUPPORTED_PDF_FILE error occurs with the following error message:
The PDF file <filename> has unsupported format and cannot be opened.
The error would occur for PDF files that are not supported by the version FineReader Engine that is being used. For instance, if the PDF file contains XFA data, it would cause this error in older versions of FineReader Engine.
Because files that would cause this error cannot be processed, it would be best to catch the ERR_FREN_UNSUPPORTED_PDF_FILE error and handle it accordingly, e.g. skip the file that caused it.