When reading math/science papers, sometime it takes more than a few days to finish reading a paper and taking note with context information is important so that its easy to come back to it at a later time. MXTNote is a free Android/IOS/Web app which helps you organize your papers and notes together for easy cross reference.
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.