Press Add Files... and Add Folders... to add the PDFs you want to number. There is no limit on the number of PDFs that can be numbered.
Once you have added the files to be numbered, press OK.
Choose the folder where you want the numbered PDFs to be saved. Choose a different folder so that your original PDFs are not overwritten by the newly numbered versions.
You can configure the format of the numbering using the next dialog.
