Alfresco 's security mechanisms ensure that only authenticated users have the permissions to access content,so even if you can send a note's repository link to an external site ,the user can't access this content through the url,because the use request the url is not authenticated .
I suggest you write a proxy servlet ,you proxy sevlet do following things.
1.users upload files to your proxy servlet,and your proxy servlet call alfresco api to add content,alfresco api will return the uploaded content node's uuid or path.
2. your proxy servlet generate an url base on your sevlet's cotextpath and node's uuid or path .when use request the url ,your proxy servlet parses the request and call alfresco api to download the file and sent it to user browser.