Which apps can magnify content on iPad?
The camera app that comes with all iOS devices with cameras can magnify images.

If you want a live preview, you can also use accessibility on the iPad to zoom the display in many, many ways, as documented by this article by Apple on the accessibility features that iOS includes.


  Zoom
  
  Zoom is a built-in screen magnifier that works wherever you are in iOS. And it works with all apps from the App Store. Turn Zoom on for full-screen or picture-in-picture view, allowing you to see the zoomed area in a separate window while keeping the rest of the screen at its native size. You can adjust the magnification between 100 and 1500 percent and access multiple filter options in either view. While you’re zoomed in, you can use all the familiar gestures to navigate your iPhone. And Zoom works with VoiceOver, so you can better see — and hear — what’s happening on your screen.


You probably don't need to invest in any third party apps until you've exhausted all of the built in capabilities of iOS and the associated hardware.