Barry
The state of Pages 5 & 6 documents are not saved with the document, unlike Pages '09 and earlier.
The Sidebar is a screen space wasting fixture in Pages 5 and 6.
This genius of a "contextual" design is particularly a nuisance with just one, let alone multiple windows open, on laptops with smaller screens. An example of the design that isn't by Apple in the last 5 years.
We used to be blessed with beautiful functionality in macOS now all we get is prettified fashion, constantly churned by a new batch of designers, most who do not seem to know what they are doing, or never use Mac Apps, let alone the ones they cripple, because somebody wants to leave their own mark on Mac Software. Just the mark, they never finish or fix what they start
atlaslab
You can not get rid of the Pages 5 & 6 Sidebar. Opening it from an email is irrelevant.
Peter