Hope you are doing well and sorry for the trouble. As described you are unable to open the PDF from NAS and getting the error
As we can see that you are using quite an old version of Adobe Acrobat Reader DC application. Please try to update the application to the latest version 21.01.20155 and reboot the computer once and see if that helps.
Also, please try to turn off the protected mode for testing (Win Only) and see if that helps. Go to Edit > Preferences > Enhanced Security > Uncheck 'Enable Protected mode at startup' , Turn off the Protected mode and uncheck 'Enhanced Security' > Click OK and reboot the computer. 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 the white list of Adobe Acrobat reader DC Go to Edit > Preferences > Enhanced Security > Add file and Add folder path and click OK > Reboot the computer and check.