Hello,
the window that comes to the front is the PDF24 Assistant (or the PDF24 Creator, if your PDF printer opens the Creator). You can switch this off in the PDF24 settings, for example with the link "Settings" at the bottom of the PDF24 Assistant:
- PDF24 Assistant: on the page "Assistant", uncheck "Bring window to foreground when a new file is added".
- PDF24 Creator: on the page "Features", uncheck "Delayed bring to front on new files".
Then click "Ok". The new files are still added to the list, but the window stays in the background.