PDF page manipulation, from 2 page per sheet to one page per sheet
Open your pdf in Preview.app. Click the toolbox icon. Click the Rectangular-Select icon. Select one of the pages. Copy it. Click File > New from clipboard. Save that file. Repeat. Now you have Untitled1 through Untitled24. Open Untitled1 in Preview. Click the menu just under the red x and choose "Thumbnails."  Drag all the files Untitled2-24 just under Untitled1 in the sidebar. Save as a pdf.