All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.
You are not sinful by reading the Quran and the books of Tafseer (interpretation of the Quran) – or other books of Islamic knowledge – from electronic copies on the phone.
The fact that you prefer reading the soft copies over the hard copies is not a disobedience and you are not sinful for this.
For more benefit, please refer to Fatwa 90896.
Allah knows best.