Hope you are doing well and sorry for the trouble. As described, the user is unable to open the PDF files and getting the error message that attached files cannot be opened.
Would you mind sharing the steps you are doing to open the PDF attachment? If the PDF file is stored on a shared network/drive please download it to users computer locally and then try to open the attachments in the PDF file and check.
Also, please try the following preference settings and see if that helps:
If it still doesn't work, please try to turn off the protected mode for testing (Windows Only). Go to Edit > Preferences > Enhanced Security > Uncheck 'Enable Protected mode at start up', Turn Off the Protected Mode and uncheck 'Enhanced Security' > Click OK and reboot the computer once. Note: Please turn on the security after testing to avoid any security risks.
You may also try to add the file and the folder path to Acrobat white-list. Go to Edit > Preferences > Enhanced Security > Add file and Add Folder path > Click OK and reboot the computer. Also, please check the network settings and group policy at your end if its a network computer.