code: func referenceCompare(fileStr string) (err error) {
  	var refFileStr, refDirStr, dirStr, baseFileStr string
  	dirStr, baseFileStr = filepath.Split(fileStr)
  	refDirStr = filepath.Join(dirStr, "reference")
  	err = os.MkdirAll(refDirStr, 0755)
  	if err == nil {
  		refFileStr = filepath.Join(refDirStr, baseFileStr)
  		err = gofpdf.ComparePDFFiles(fileStr, refFileStr, false)
  	}
  	return
  }
docs: // referenceCompare compares the specified file with the file's reference copy
  // located in the 'reference' subdirectory. All bytes of the two files are
  // compared except for the value of the /CreationDate field in the PDF. This
  // function succeeds if both files are equivalent except for their
  // /CreationDate values or if the reference file does not exist.