Page width fills the window on a phone, without making the page huge on a big screen.
While you move things, they line up with the page's edges and middle and with each other, and a guide line shows the match. Hold Alt (Option on a Mac) while dragging to move freely.
On: keep scrolling down past the bottom of a page to go to the next one (and up past the top for the one before). Scrolling over the page thumbnails always goes from page to page.
On: after you place a red X, a checkmark, a highlight, a shape or a form field, the same tool stays ready for the next one - handy for marking a whole page. Off: PDFEDTR goes back to Select, so you can adjust what you just placed. Step numbers always stay ready, since they usually come several in a row.