candeed

maybe you should try Amazon Link Plugin.

“…This plugin is used to quickly add Amazon products to your WordPress posts and pages. The products can be simple text links, thumbnail images, fullsize images, complex templates or full-blown Amazon flash widgets.

The product links that the plugin generates can be for any of the Amazon domains…”

WooCommerce has simple affiliate product feature (for referring external products), but I’m not sure if it can handle specific requirement as what you need.

@VanGill Media

When adding a free product, treat your free product as ‘external product’..
Then ‘add to cart’ button will dissapear,
and there will be an option to rename the button to any name as you like..
E.G. You can name it ‘download’ and then link the button to the free file.

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Anyway, if you want to change ALL ‘add to cart’ buttons on entire site..
just open function.php in your theme folder.. add this text:

add_filter('single_add_to_cart_text', 'woo_custom_cart_button_text');

function woo_custom_cart_button_text() {
return __('My Custom Button Text', 'woocommerce');
}

before this line :

/*-----------------------------------------------------------------------------------*/
/* Don't add any code below here or the sky will fall down */

Hope it help..