We are sorry for the trouble. As described you are unable to save the PDF and gets the 'Unexpected Error'
We need some more information for a better understanding:
Is this a behavior with a particular PDF file or with all all the PDFs? Please try with a different PDF file and check.
What is the workflow/steps yo are doing to save the PDF file?
Please share the screenshot of the error message that you get while saving the PDF.
Are you trying to save the PDF on a shared network/drive or locally on your computer system?
What is the version of the Adobe Acrobat DC you are using? To check the version go to Help > About Adobe Acrobat DC and make sure you have the latest version 20.009.20074 installed. Go to Hel[p > Check for Updates.
Are you on Mac or Windows machine and what is the version of the OS?
For testing purposes, you may try turning off the protected mode and see if that makes any difference. Go to Edit (Windows), Adobe Acrobat DC (Mac) > Preferences > Enhanced Security > Turn Off the protected Mode and uncheck the Enhanced Security > Click OK and reboot the application and check. Note: Please turn on the security after testing.