https://community.adobe.com/questions-12/acropdfimpl-dll-hangs-during-unloads-1512673
AcroPDFImpl.dll Hangs During Unloads | Community
Starting with the 7/31/17 cut, whenever the AcroPDFImpl DLL_PROCESS_DETACH event fires, the module puts the thread to sleep and causes 2-3 second hangs...
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