In a formal invitation issued by a Ph.D. is the correct title Dr. or is that for medical doctors only?

The title of Dr. is specific to those that hold a doctorate, in any subject, and is not directly related to the medical profession. Medical doctors are called Dr. because they hold a doctorate in medicine. Other doctorates (e.g. Ph.D.) should also be addressed as Dr. for the same reason. Some individuals confuse this issue over the fact that in the English speaking world the title doctor (profession) is used in place of the more specific title physician and thus associate Dr. with only medical doctors. This is, however, incorrect and all doctorates are Drs.