You can easily do this with the built-in tool of iOS.
- Go to Settings of iOs
- Drag down on the list to bring out the search bar. Type Zoom (see image below)
- Turn on "Zoom"
Now, you can double tap using 3 fingers to bring out the magnifying glass.
Double tap and hold your 3 fingers, then slide up to zoom in.
To resize the magnifying glass, you can triple tap using 3 fingers.