Help! Can open pdf in Acrobat DC BUT unable to edit and 'save as'

I can make pdfs from applications e.g. InDesign.

I can open the pdf in Acrobat DC

BUT, I am unable to edit the pages or "Save as"

As soon as I start to do this I immediately see a flashing blue circle which becomes my cursor.

If I try to save as I then see what should be the window for specifiying the name of the file and the destination for where I want to save the file. But this information is NOT there and this window just flashes 'white' and empty.

I do not have Microsoft office on this computer and I have updated all the CC applications I use.  I've uninstalled Open Office.

I have also tried uninstalling Acrobat DC and reinstalling it.  But the result if the same.

Huge thanks in advance if anyone can help me to resolve this problem!