Is this happening in macOS? It looks like is macOS.
If it is macOS, I can think of a similar workaround for this issue that was addressed yesterday by ACP Karl Heinz Kremer to a MS Windows 10 user.
It seems like the file viewer (in macOS the "Finder" file viewer) hogs certain applications as if they're used by another program when you have the "Preview Pane" enabled.
In MS Windows 10, openWindows Explorer, click on the "View" tab and clcik once on "Preview Pane" to disable.
In macOS , open the Finder, click on the "View" in the menu bar and select "Hide Preview Pane" (or "command"+"shift"+ "P" keyboard combination).
++ Adding to the discussion
What is the version of the Adobe Acrobat DC You are using? To check the version go to Help > About Adobe Acrobat and make sure you have the latest version 20.12.20043 installed. Go to Help > Check for Updates.
Is this a behavior with a particular PDF file or with all the PDFs? Please try with a different PDF file and check. If its a file specific issue please share the file with us for testing.
Please upload the file to the document cloud https://documentcloud.adobe.com/link/home/ generate the link and share it with us.
With your file opened in Acrobat just click on the share link icon (it has a cloud and paper clip icon--- far right corner of the Main Toolbar).
When you click on that icon it will upload a copy of your file automatically to the Document Cloud.
A checkmark iwill turn green when the link is ready and copied into temporary memory (the clipboard).
So just reply back here by all you have to do is right-click and paste the link in the chatbox messaging window that were using, and click "Post"
Why want you change the security settings?