Always try to find a volunteer to read over you work before you consider it completed. You can usually return the favour and then both of you benefit. There are three. It counts the number of words with more than two syllables in your document and uses that to gauge reading difficulty. This is, like other reading measures, a limited assessment tool.
People can often read difficult words if the vocabulary is within their experience.
For instance, many slower readers with developmental disabilities can read developmental disabilities. Finally, do add pictures if you can find ones which are clear, easily understood and applicable to the text. They lighten up a document and give readers a clue to the written content. Test them as you would any written text, by asking your intended readers what they think the pictures mean. People may not understand how long it has taken you, but they will appreciate the results.