The PDF features of Bookends for iOS are powered by PSPDFKit. The same engine is used in a free app called "PDF Viewer" from the PSPDFKit team.
If you want some feature to appear in future releases of Bookends, please download PDF Viewer.
If PDF Viewer does not have a feature you would like please send a request fo PSPDFKit support explaining why do you need this feature. If they agree they will try to implement it. If they do, it will likely become available on all apps that use their PDF engine, including Bookends for iOS.