If you are using an image, you can choose to aspect fill, scale to fit, or tile if you have a repeating pattern like in this example. Just select what kind of fill you want to change the page background. You get the same kind of options as with any object’s background color fill, image fill, or gradient fill. You will now see a Page Layout header in the sidebar, with the options for setting the background just below it. With the document body disabled, you can now go back to the Format menu - make sure not to select any object. Ideally, you should remember to uncheck this box when you first make your document and you are staring at a blank page. You can undo, there is no good way to migrate over. Pressing this will show a scary warning that conversion requires deleting any body text or inline elements this means any text not in a text box will be deleted. On Mac, to convert any open document into page layout mode, open the Document sidebar by pressing the icon in the top right of the toolbar. iOS does not currently have this capability. Now, on the Mac, it is possible to change any document into a page layout mode. Page background controls are only available on page layout documents, as you can set a page background for every distinct page that you add. Page layout option is best for certain types of projects - posters, newsletters - for purposes that need more fine-grained control over the design. The ‘word processing’ option is the general default.

How to change background color on pages