Gerando thumbnail de arquivos PDF

// read page 1
$im = new Imagick('test.pdf[0]');

// convert to jpg
$im->setImageColorspace(255);
$im->setCompression(Imagick::COMPRESSION_JPEG);
$im->setCompressionQuality(60);
$im->setImageFormat('jpeg');

//resize
$im->resizeImage(290, 375, imagick::FILTER_LANCZOS, 1); 

//write image on server
$im->writeImage('thumb.jpg');
$im->clear();
$im->destroy();