I suddenly have begum receiving an error message that tells me Acrobat Pro 11 is not compatible with my version of Windows. I have made no software changes. Windows won't allow me upgrade version 7 leaving me unable to open existing PDF files or to complete online PDF forms. Any help will be appreciated.
As per the issue description mentioned above, you are getting incompatibility message about Acrobat XI on your windows machine, is that correct?
Could you let us know the operating system installed on the machine? You may also refer to the following help document to check Acrobat XI compatibility with windows operating systems: Adobe Acrobat system requirements
You may also download the previous version of Reader XI to view pdfs and check if that works for you. Adobe - Adobe Acrobat Reader DC Distribution (choose Windows XP as operating system).
Let us know if you need any help.