Hope you are doing well and sorry for the trouble. As described the e-mail attached PDF file is opening in MS Edge.
Would you mind sharing the workflow/steps you are doing to open the e-mail attached PDF file? Please try to download and save the PDF file to your computer locally and then try to open it with Adobe Acrobat/Reader DC application and check if that works for you.
Also make sure you have the latest version of the application. Go to Help > Check for Updates and reboot the computer once.
Let us know if you experience any trouble and need more help