An iOS app is available from the Apple App Store. Users can collect and organise references, read and annotate PDFs, and synchronise with the desktop library. Requires iOS 14.7 or later.
If your smartphone is not an iPhone, or if it does not have IOS 12, you can bookmark this page, enter an ISBN and the book reference will be added to your library. You must have an account on the Zotero website and to enter your login and password the first time you use this feature.
Zotfile lets you sync your PDFs with your Android tablet; you can annotate and highlight them on the tablet, and extract the annotations into a Zotero note when you retrieve the files from your tablet.
Zandy (not free) provides access to the Zotero library; users can edit, sync their library and work offline.
ZotEZ2 lets you sync a Zotero library to an Android device.
Zoo for Zotero is an Android application for viewing Zotero libraries, adding and editing notes, and adding items by ISBN or DOI.