Splits pages in half. A page (or pages) can be split horizontally, vertically or depending on the orientation defined for the page (example: portrait or landscape). You can also select the page box you want to base this split on.
The following screenshot shows a page before it is split and after it is vertically split.

To split all the pages vertically in half, select all pages vertically.
To split only portrait pages in half, select only portrait pages.
To split only landscape pages in half, select only landscape pages.
To split the pages with the least used orientation, choose automatic.
First, the PDF is analyzed for portrait and landscape pages and then the pages are split. Landscape pages will be split vertically and portrait pages will be split horizontally. All other pages will be untouched.