Issue highlighting text and saving changes to PDFs

Hi

I am using Windows 10 Pro and Adobe Reader XI. I have never had problems highlighting text before but now I am having trouble and when I highlight text and click save, instead of saving it comes up with a dialogue box to 'save as', when I click save it asks me if I want to replace the file, when I say yes I get the following message:

the only restrictions on the document are on page extraction and document assembly.

Any ideas what is going wrong?

thanks