You need to encode the parameter data when passing it via a URL, TIdHTTP
will not encode the URL for you, eg:
http.Get(TIdURI.URLEncode('http://localhost/Service/Messaging.svc/SendReports/PDF?xml=<?xml version="1.0"?><email><message><to>email@internal.com</to><from>from@internal.com</from></message></email>&id=42&profile=A1'));
Or:
http.Get('http://localhost/Service/Messaging.svc/SendReports/PDF?xml=' + TIdURI.ParamsEncode('<?xml version="1.0"?><email><message><to>email@internal.com</to><from>from@internal.com</from></message></email>') + '&id=42&profile=A1');