Preview not saving edits

I have been highlighting an important document over the last few weeks, and have been highlighting as I go, and have only just realised it has not saved half of the highlights I've made. For each page, there are only highlights visible on the top third of the page, the rest remains unhighlighted. I know I highlighted them, but I cannot see them. I have tried reverting the documents, but EVERY SINGLE copy, after I have highlighted it, once it has saved it seems it deletes the bottom two third of hard work off of every single page.

Can someone please help me get the highlighted bits back for all of the edits I have made - its taken me two weeks to get through 70 pages and only halfway done.