acrobat reader "an incorrect structure was found in the PDF file"

Acrobat Reader was "updated" on 24th March, but appears to have been corrupted. 

I have just spent an hour editing a student's script. When working on the comments, I get the error above. When I try to save it, I get

"The document could not be saved. A number is out of range"

What is happening? Why is Adobe corrupting Reader?  Or is it Microsoft, which has just reset Windows with all the normal consequent damage?

How can I rescue the work I have done?

Thank you