Digital devices are getting smaller and smaller. This is great for portability, but not so great for the eyes.
Smaller devices mean smaller screens and that means the print is pretty tiny. The tendency is to hold the device closer to try to see the smaller text, but this requires greater effort to focus and keep the eyes aligned.
Also, depending on the screen resolution, holding the screen closer to the eyes can cause pixilation of characters, which blurs the text and contributes to eyestrain.
To reduce the risk of eyestrain from smaller screens, adjust the print to a larger size, increase the brightness of the screen to match your surroundings and take frequent breaks when reading long passages of text.
For sustained reading, use a tablet or notebook device and hold it farther from your eyes for greater viewing comfort. Or better yet, go outside and take a walk.