As explained in the Report Book
help article, the Page Settings button is disabled by design when displaying a report book. That is because we cannot know what the PageSettings of the various report in the book are.
In such case the PageSettings of the first report are taken into account and if all of your reports have been set as A5, then they would be printed as A5 if your printer is recognized to support this format. That is what we have tried to explain in our previous post - that it is the printer settings that define the default paper kind. Please review our explanations there and try setting the MS XPS viewer as default printer to see how it behaves then.
On a side note, if feasible for your scenario, you can try combining your reports into a single one, by using the SubReport
item. This way only the paper kind of the main report would be taken into account.
the Telerik team