Hi

I don't know why pdfs hang up, but you definitely can't refer a dropdown list to another workbook if it's closed. But what you can do, is fetching the data from the closed workbook to a sheet of your workbook you are working with by ordninary formulas. This range then you can use for your dropdown, and excel will accept this.

With your extern references in your workbook, excel will ask on opening, if the references shall be updated or not.