ClaroPDF is an accessible, reading and study PDF tool that allows you to annotate, mark up, save, and read aloud PDF files which are then still fully compatible with Adobe Reader and other popular PDF apps.
Some people with low vision find light text on a black background and other color inversions to be more accessible than black text on a white background.
Magnifier is an iOS feature that works like a digital magnifying glass. It uses the camera on your device to increase the size of anything you point it at, so you can see the details more clearly.