I have installed the plugin and changed the file permission of the productapi.php to 644, but when I navigate to mydomain.com/productapi.php, get page not found. Why is it so? I see the file at the root of my folder.
The permission 644 works for about 95% of our users. Few users use 755, 775 or 777. Just try any of these permissions and visit yourdomain.com/productapi.php to if the xml is generated.
You may also contact your hosting guys to help you.