Hope you are doing well and sorry for the trouble. As described, you are unable toopen a second file and save the binder and getting the error message 'working on it'
Since you have the latest version of the Adobe Acrobat DC, please try to reboot the computer once and see if that helps.
If it still doesn't work, please try to repair the installation (Windows Only) and reboot the computer again and check. Go to Help > Repair Installation.
Do you get any error message when you try to open the second file? If yes, please share the screenshot of the same. Also, if the file is located on a shared network/drive, please try to save it locally on your computer and then try to open and check.
Please try the following preference settings and see if that helps:
- Go to Edit > Preferences > General > Uncheck 'Show online storage when opening file' and 'Show online storage when saving file' > Click OK
You may also try to turn off the protected mode for testing (Windows Only). 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 once.
Note: Please turn on the security after testing to avoid any security risks.