We are sorry for the delay in response and the trouble caused.
As described when you open the PDF everything is grey black. Is this a behavior with a particular PDF file or with al the PDFs you open? Please try to open a different PDF file and check. If its a file specific issue please share the file with us for testing.
Also, please share the screenshot of the issue for a better understanding. And make sure you have the latest version of the application 20.006.20042 Go to Help > Check for Updates.
You may also check the preference setting. Go to Edit (Windows), Adobe Acrobat/Reader DC (Mac) > Preferences > Accessibility > Select, 'Replace document Colour' and under custom color make sure page background is set to white and document text is set to black color. > Click OK