Diffchecker, is a free online tool that does a fine job of comparing differences between two text or PDF files.
See this comparison of the official Sedona Conference Proclamation on the left, and my edit of it on the right.
Altered words are highlighted on both sides.
Diffchecker can also help you compare multiple versions of an image. It will show the differences by overlaying two images:
. . . and even better by letting you slide back and forth between them.