After you've completed converting a file, you may want to convert another one immediately after.
Of course, you can use the Open options to simply open another instance of PDF2XL with the new PDF, but doing so will lead to multiple PDF2XL windows cluttering up your workspace.
Instead, you can close just the current PDF file without closing the whole PDF2XL program.
It's as simple as clicking the red 'x' in the top-right corner of the PDF2XL window.
Be sure to avoid clicking the one at the uppermost corner, as this will close the whole program.