...
File tempFile = File.createTempFile( "pd4ml", ".pdf" );
java.io.FileOutputStream fos = new java.io.FileOutputStream( tempFile );

PD4ML pd4ml = new PD4ML();
pd4ml.setHtmlWidth( 800 );
pd4ml.useAdobeFontMetrics( true );

pd4ml.useTTF( "/windows/fonts", true );

pd4ml.render( "http://www.yahoo.com", fos );

String pdfFile = tempFile.getAbsolutePath();
String viewcmd = 
    "\\Program Files\\Adobe\\Acrobat 6.0\\Reader\\AcroRd32.exe " + pdfFile;

Runtime.getRuntime().exec( viewcmd );

...