MXTNote is a free PDF reader and note taking app, its a management system with emphasis on links and cross reference.
The basic objects are grouped into the following sections
Links can be created automatically when you reference something from your notebook to original documents. Additional links can be created link in the simple notes layer to capture more relations.
MXTNote is available as web app and android/IOS app. To use web app or mobile app in online mode, either sign up with free MXTNote account, or Sign in with google/apple. If you don't want to sign in, try the mobile app version in offline mode first.
Start with small note/bookmark, later expand to long multi-page note
When you are online, your documents and notes are automatically synced.
When you are offline, your changes are stored in your device and you can sync them when you are online again.