$im = new Imagick();
$im->setResolution(300,300);

$im->readImage('mypdf.pdf');
$im->setImageUnits(imagick::RESOLUTION_PIXELSPERINCH);
$im->adaptiveResizeImage(707,1000);
$im->setResolution(300,300);
$im->setImageFormat( "jpg" );
$originalImageFile = 'myjpg.jpg';
$im->writeImage($originalImageFile);