Strange, actually Windows should bring the app window back to the first screen. Try to connect the second screen, then open pdf24 reader, move the window back to the first screen and close pdf24 reader there. this should actually help. If not, try to delete the following path in the registry via regedit:
HKEY_CURRENT_USER\Software\PDF24\PDF24 Reader\Persistent_Options
This stores the position of pdf24 reader on screen. Restauration on next start will not be performed if this key is deleted.