My version of Acrobat has regressed from 8.1.3 to 8.0. Why?

I've been using Acrobat 8 for years now; it has always worked fine, even if I have changed OS at least three times over the past 6 years. I now have Windows 10.

Today I tried to open a PDF file from internet, and strangely received a message telling me that my version of Adobe Acrobat needed to be updated. When I checked the properties of the program, I discovered that it was version 8.0, whereas the last time I had checked it was version 8.1.3. I tried repairing the program, but was told that the Adobepdf.dll file was missing. I have downloaded it and placed it in the Adobe folder, but this makes no difference. I cannot update the program.

Does anyone know why my version of Acrobat has gone back to 8.0, and how I can fix this short of doing a full reinstall?