A response content stream (or bytes) is unidirectional. If you read the content, you have to provide a new response object with the same content. Here is an example:
Client.OnResponseReceived := procedure(Req: THttpRequest; var Resp: THttpResponse)
begin
Resp := THttpCachedResponse.Create(Resp);
var TextResponse := TEncoding.UTF8.GetString(Resp.ContentAsBytes);
end;