def output_file(file = nil)
      if file
        output_filename = file.name
        if !@config.output_filename.nil? && @files.length == 1
          output_filename = @config.output_filename
        end
      else
        output_filename = Time.now.to_s.split(' ').join('_')
        output_filename = @files.last.name if @files.length == 1 || @config.merge
        output_filename = @config.output_filename unless @config.output_filename.nil?
      end

      ::File.join(output_dir, "#{output_filename}.pdf")
    end
def output_file(file = nil)
      if file
        output_filename = file.name
        if !@config.output_filename.nil? && @files.length == 1
          output_filename = @config.output_filename
        end
      else
        output_filename = Time.now.to_s.split(' ').join('_')
        output_filename = @files.last.name if @files.length == 1 || @config.merge
        output_filename = @config.output_filename unless @config.output_filename.nil?
      end

      ::File.join(output_dir, "#{output_filename}.pdf")
    end
Generate the name of the output file
 @return [String]
 @param [Gimli::MarkupFile] file optionally, specify a file, otherwise use output filename