This is a security setting Adobe forces on the app. There is a simple fix. However, sometimes updates turn this back on.
THE FIX
- Open Adobe Acrobat.
- Click EDIT then PREFERENCES at the top left.
- When the preferences window open, click SECURITY (ENHANCED) on the left side.
- Find SANDBOX PROTECTIONS on the right and make sure PROTECTED VIEW is OFF or DISABLED.
- Click OK and you should be able to open multiple PDFs at the same time.
The image below might be slightly different from your screen, but the details are the same.