I have been testing your sample app, and we have found a problem highlighting long files.
The file has more than 900 pages and when we select text and highlight it the page is modified right but from this moment on we cannot see the rest of the pages of the file and also when we close the file the highlight is not saved.
With shorter files it works as expected.
To discard licence problems we have tested the same file with our app and our licence and the behaviour is exactly the same.
I highlight a word. It gets highlighted fine. When i drag to next page it does not show anything. When i close the file it shows the prompt to save it and when i say yes it is not saved because when i open again the highlight does not exist.