Choose Select All in the Edit menu to select everything on the current page in the active area.
All zones are selected if the Page Image panel is active.
All text and retained graphics are selected if the Text Editor is active.
Typically you select all zones on a page to delete them and draw your own zones. You could also select all zones to change them all to a different zone type in one move. Typically you select all text on a page to change its font, font size or alignment or to copy it to Clipboard.
Choose Select All in the Edit menu when the Document Manager or Thumbnails are active to select all pages in the document.
Select All is also available in the shortcut menus.