Source

File: wp-includes/class-wp-image-editor-imagick.php

protected function pdf_setup() {
		try {
			// By default, PDFs are rendered in a very low resolution.
			// We want the thumbnail to be readable, so increase the rendering DPI.
			$this->image->setResolution( 128, 128 );

			// When generating thumbnails from cropped PDF pages, Imagemagick uses the uncropped
			// area (resulting in unnecessary whitespace) unless the following option is set.
			$this->image->setOption( 'pdf:use-cropbox', true );

			// Only load the first page.
			return $this->file . '[0]';
		} catch ( Exception $e ) {
			return new WP_Error( 'pdf_setup_failed', $e->getMessage(), $this->file );
		}
	}