Upload File Restriction

This is the upload file code and i want users to upload only PDF file.
Can anybody help ??

<?php
/**
 * File Upload field
 */
class Product_Addon_Field_File_Upload extends Product_Addon_Field {

	/**
	 * Validate an addon
	 * @return bool pass, or WP_Error
	 */
	public function validate() {
		if ( ! empty( $this->addon['required'] ) ) {
			foreach ( $this->addon['options'] as $option ) {
				$field_name = $this->get_field_name() . '-' . sanitize_title( $option['label'] );

				if ( empty( $_FILES[ $field_name ] ) || empty( $_FILES[ $field_name ]['name'] ) ) {
					return new WP_Error( 'error', sprintf( __( '"%s" ist ein Pflichtfeld.', 'woocommerce-product-addons' ), $this->addon['name'] ) );
				}
			}
		}
		return true;
	}

	/**
	 * Process this field after being posted
	 * @return array on success, WP_ERROR on failure
	 */
	public function get_cart_item_data() {
		$cart_item_data           = array();

		foreach ( $this->addon['options'] as $option ) {

			$field_name = $this->get_field_name() . '-' . sanitize_title( $option['label'] );

			if ( ! empty( $_FILES[ $field_name ] ) && ! empty( $_FILES[ $field_name ]['name'] ) ) {
				$upload = $this->handle_upload( $_FILES[ $field_name ] );
				if ( empty( $upload['error'] ) && ! empty( $upload['file'] ) ) {
					$value  = woocommerce_clean( $upload['url'] );

					$cart_item_data[] = array(
						'name' 		=> $this->get_option_label( $option ),
						'value'		=> $value,
						'display'	=> basename( $value ),
						'price' 	=> $this->get_option_price( $option )
					);
				} else {
		    		return new WP_Error( 'addon-error', $upload['error'] );
		    	}
			} elseif ( isset( $this->value[ sanitize_title( $option['label'] ) ] ) ) {
				$cart_item_data[] = array(
					'name' 		=> $this->get_option_label( $option ),
					'value'		=> $this->value[ sanitize_title( $option['label'] ) ],
					'display'	=> basename( $this->value[ sanitize_title( $option['label'] ) ] ),
					'price' 	=> $this->get_option_price( $option )
				);
			}
		}

		return $cart_item_data;
	}

	/**
	 * Handle file upload
	 * @param  string $file
	 * @return array
	 */
	public function handle_upload( $file ) {
		include_once( ABSPATH . 'wp-admin/includes/file.php' );
		include_once( ABSPATH . 'wp-admin/includes/media.php' );

		add_filter( 'upload_dir',  array( $this, 'upload_dir' ) );

		$upload = wp_handle_upload( $file, array( 'test_form' => false ) );

		remove_filter( 'upload_dir',  array( $this, 'upload_dir' ) );

		return $upload;
	}

	/**
	 * upload_dir function.
	 *
	 * @access public
	 * @param mixed $pathdata
	 * @return void
	 */
	public function upload_dir( $pathdata ) {
		global $woocommerce;

		if ( empty( $pathdata['subdir'] ) ) {
			$pathdata['path']   = $pathdata['path'] . '/product_addons_uploads/' . md5( $woocommerce->session->get_customer_id() );
			$pathdata['url']    = $pathdata['url']. '/product_addons_uploads/' . md5( $woocommerce->session->get_customer_id() );
			$pathdata['subdir'] = '/product_addons_uploads/' . md5( $woocommerce->session->get_customer_id() );
		} else {
			$subdir             = '/product_addons_uploads/' . md5( $woocommerce->session->get_customer_id() );
			$pathdata['path']   = str_replace( $pathdata['subdir'], $subdir, $pathdata['path'] );
			$pathdata['url']    = str_replace( $pathdata['subdir'], $subdir, $pathdata['url'] );
			$pathdata['subdir'] = str_replace( $pathdata['subdir'], $subdir, $pathdata['subdir'] );
		}

		return apply_filters( 'woocommerce_product_addons_upload_dir', $pathdata );
	}
}