Hope you are doing well and sorry for the trouble.
Is this an issue with a particular PDF file or with all the PDFs? Please try with a different PDF file and check. If the file is stored on a shared network/drive, please download it to your computer locally and then try again.
As you have the recent version of the application and tried some troubleshooting steps with no luck, please try
removing the email account you have in Adobe Reader and then add it back and make it default. Navigate to Edit>Preferences>Email accounts>Remove the current email account.
Then reboot the machine and add the email account again from Edit>Preferences>Email accounts.