code: public boolean accept(File f)
      {
  	if (f.isDirectory())
  	{
  	    return false;
  	}
  	String name = f.getName().toLowerCase();
  	return name.endsWith(".pdf");
      }
docs: Judges whether a file is a PDF document or not
  
  @param f
  file to check
  @return a boolean value: yes/no