Based on your post, I understand that you want to the user can receive notification when the file been modified.
You could set alert in the document library. When you modify the file, email will be sent to the user.
1.Click three dots -> Alert me.
2.In the Send Alerts To section, fill in the user account.
To track changes in the file:
1.Click the File tab -> Info -> Version History.
2.In the review tab -> Track changes.
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