* Ontex shares down 19 percent (Adds share price move, analyst comment): (Reuters) - Belgian diaper maker Ontex (ONTEX.BR) has rejected a reduced bid from French private equity firm PAI Partners as undervaluing the company, sending its shares down as much as 22 percent on Monday. Ontex said both sides now considered the matter closed. PAI was not immediately available to comment. In July, PAI had lifted its offer for Ontex to 27.5 euros per share, or about 2.3 billion euros ($2.7 billion), after its initial proposal was rejected.