Access to Insight is an extensive website with English translations of Buddhist sutras from the Pali Canon of the Theravada tradition. It is probably one of the largest collections of Buddhist sutras on the Internet. There is an app that contains the entire material of the website for convenient use without an internet connection. (Note: this app was not written by me!)
There is also a downloadable “offline edition” of the website which is offered as a zip file and which can be viewed on most devices and computers after unpacking.