It would be great if you could invert the selection, e.g. so you can go through a document and just select the few pages that you want, then delete the rest.
This feature would be very easy to implement but there is already a simpler way to do what you describe.
1) Select the pages you want to keep.
2) Open a new pdfshuffler instance.
3) Drag and drop the selected pages into the new empty pdfshuffler window.
Anyway I can add a shortcut for selection inversion in the next release.
> Drag and drop the selected pages into the new empty pdfshuffler window.
Not possible when running pdfshuffler in windows unfortunately :(