What does it mean when the CA19-9 level goes back to the normal range when someone has pancreatic cancer? Is this a sign of remission?

Well.... If it was elevated before surgery and now is normal, I would say it is a very good thing BUT, that as of itself does not mean it is in remission or that it will stay that way. Cancer can exist in microscopic form and often there is no way to know for sure, be it with labs or even with scans. Such patient will need long term follow up and maybe even consider post operative therapy. Best 2U!
Promising sign. You mentioned the CA 19-9 was elevated before treatment; the key here is whether the patient was jaundiced at the same time. CA 19-9 can be a good cancer marker, but sometimes jaundice alone can make it high. Normalization of CA 19-9 is most promising if there was no accompanying jaundice, but either way it's better than a persistently high level after treatment.