All individual pages of an issue are saved as PDF files on Android under the path „...\Android\Data\com.pressmatrix.emk\files\Editions\<Ausgaben-ID>“. In addition, there are the thumbnail images of the individual pages for the page overview, as well as all files of all widgets. No distinction is made between preview or normal editions; the apps are all treated in the same way. If the user deletes an issue in the app, the folder with all these files belonging to this issue is removed from the file system. So no "rubbish" is left on your device.
On iOS devices it is handled in the same way as on Android. Nothing is stored by the app in the download folder.