Polar supports mobile devices for reviewing your annotations. We don't currently support full reading and creating new annotations on mobile due to user-interface complexities 1
Polar mobile can be used (and installed) by just visiting the site and logging into the web application on your mobile device.
Polar mobile isn't available on the Apple App Store or the Android Play Store but is a "progress web application" (PWA) that can be installed just like any local app.
Both iOS and Android support PWAs and can be installed by clicking "Add to Home" after you've loaded the app on your mobile device.
For iOS (iPhone) you will have to install via Safari and on Android you will have to install via Google Chrome.