Logseq’s PDF reader/annotator only works on the desktop app (meaning Windows, Linux, macOS).
This is one of the most popular questions about the mobile app (whether it’ll support the PDF reader, plugins, themes, etc.). Unfortunately, there’s no mobile developer contributing to the project at the moment, so there’s not much happening on the mobile front. We’d need someone with some serious mobile UI/UX experience on the team to improve the iOS and Android apps.