WooCommerce appends a unique string to the filename of uploaded files to enhance security.
It is recommended to be turned on and will be turned on by default, but it’s not required if you have already properly configured your upload directory. No filename or links will be affected when you turn it on, only files uploaded when this setting is checked will have a unique string added to them.
This is setting is fully backwards compatible and can be turned off-on at whim without affecting any already generated file links.
You can access this setting by going to WooCommerce > Settings > Products > Downloadable products.