Unable to add new comments to a PDF since updating

Hi, Just wondering if anyone else has this problem or any idea of how to solve it - I have been working on a PDF document for a few weeks and adding comments to it. Before today, these comments have saved successfully each time I save the document, and then I am able to resume working on the document and adding changes the next day. My computer had many updates last night and today, and now I am not able to add comments to my PDF anymore.

This message appears when I open the document 'This document enabled extended features in Adobe Acrobat Reader. The document has been changed since it was created and use of extended features is no longer available. Please contact hte author for the original version of this document'.

I then checked to see if I could save comments on the original PDF, reopen it, and add more comments, in order to finish my work by using two separate PDFs, but this didn't work. It seems that once I save and exit the PDF, the comments I made are saved, but I can no longer edit that PDF when I reopen it.

I'd appreciate any assistance at all - it's driving me crazy!!

Thank you!