iOS has inbuilt features for Accessibility. The form can have text that is too small for some donors. You can enlarge it for them and still use the app by using the Accessibility features.
The feature is described here https://www.apple.com/au/accessibility/ios/ and this is how it works in practice.
[Apple]: Zoom is a built-in magnifier that works wherever you are in iOS, from Mail and Safari to the Home and Lock screens. And it works with all apps from the App Store. A simple double-tap with three fingers instantly zooms in 200 per cent, and you can adjust the magnification between 100 and 500 per cent.
[Waysact]: For example you're on the signature page and the donor can't read everything they need to, like the summary details on the top left hand side of the page. Here I double-tapped with three fingers on the summary information and it zoomed straight in.
[Apple]: While you’re zoomed in, you can still use all of the familiar gestures to navigate your device.
[Waysact]: So, for example you might need to zoom in to the signature to make it easier to write on the screen (it really is!). See below: