Normally schemas are meant to stay static. They are hard coded to define a dynamic xml file. For instance, the element gender in a schema could have the enumerations Male and Female. Yet for a drop down that would change a lot, you wouldn't put it in the schema but the xml file the schema points to.
It sounds to me like you need to map your drop downs to an xml. Then before you publish your pdf, you would import the latest xml and all your drop downs would be populated with the correct items.
Look up dynamic properties for drop downs.