The ann file extension is related to the Microsoft Windows Help system used for browsing help documents. The ann file stores annotations. An ann file extension is associated with the Microsoft Windows Help tool.
To open an ann file, use Windows Help. Unfortunately, it seems the WinHelp is no longer supported by Microsoft.
When you double-click an ann file to open it, Windows checks the filename extension. If Windows recognizes the extension, it opens the file in the associated program. When Windows does not recognize an extension, you receive a message to select an app to open the ann file.
To associate the ann file extension with a program in Windows, open Settings, click on System, select Apps, and choose a program to open ann files.
If Windows cannot open an ann file, the issue may be file corruption, incorrect registry entries, accidental deletion of the file description, incomplete installation of the supporting application, or malware infection.
Common issues opening ann files include lacking the required software (e.g. ABBYY Lingvo Dictionary), or not having the ann file extension associated with a program. To fix, install software that supports ann files, and associate ann files with that software.
ABBYY Lingvo
Microsoft Windows Help
Older Windows versions