to be clear, make sure there is no 0-expGraph-Last used.ois file in UFF’s Themes subfolder.
new graph, open expGraph dialog from File menu
set image type to PDF, click the triangle button, select save as default, close the dialog
reopen the dialog from File menu “Open Dialog“ entry.
--> <default> theme is showed up as dialog theme,
==> but image type is EPS
click the triangle button, select default
? image type is changed to PDF
Fixed in Origin2023b.