What happened to my sticky notes in preview?

I spent a lot of time adding comments to a pdf file. I used Preview. Now when I go to the document, all of the yellow squares are there, but there is no content inside. In fact, when I double click on the sticky notes, the boxes don't even open. But if I click on Annotate in the toolbar and click on the yellow box, it opens, but it's blank.

I put in a lot of work on the two documents and now it's all gone. It's the first time I've ever tried doing comments in Preview (I've always used Adobe Reader and never had a problem. I've been trying to stick with the Mac OS tools instead of downloading readers and such, but now I'm having second thoughts about this.

Normally I would do tracking in Word for comments/corrections like this, but I had to work with a pdf because the original document was generated in Pages and I can't seem to open Pages.

Are my comments lost forever?