Pfizer Shares Fall on Analyst Downgrade  NEW YORK (Reuters) - Shares of Pfizer Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=PFE.N target=/stocks/quickinfo/fullquote"&gt;PFE.N&lt;/A&gt; closed  down more than 2 percent on Monday after an analyst lowered her  investment rating on the world's largest pharmaceutical  company's stock.