The following are properties for ContentDownloadHandler.
public String downloadErrorMessage {get; set;}
Type: String
This message is used if a redirectUrl is not provided. If the download is not allowed, Salesforce will throw a ContentCustomizedDownloadException exception that contains the downloadErrorMessage.
public Boolean isDownloadAllowed {get; set;}
Type: Boolean
public String redirectUrl {get; set;}
Type: String
The URL must be a valid relative URL. For example, the redirect can be a custom Visualforce page such as “/apex/IRMControl”. URLs with no path, such as “www.domain.com”, will result in an InvalidParameterValueException.