: Because Acrobat 9 no longer receives security patches, using it to open PDF files from the internet poses a significant risk to your system.

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Adobe explicitly lists only and versions 2017 and 2020 as officially supported on Windows 11.

Windows 11 has stricter memory management. Acrobat 9’s "Protected Mode" (sandboxing) is incompatible with Windows 11’s Core Isolation and Memory Integrity features. You would likely need to disable Windows Security features to run it – a dangerous move.