How does the Win7 system cancel the file association?

How does the Win7 system cancel the file association? ：

1, click Start Menu-run, and enter "regedit" in the running column to press ENTER;

2, enter the Registry Editor, click on the order to open: "Hkey_current_usersoftwaremicrosoftwindowscurrentversionexplorerfileexts.pdf;

3, the "Openwithlist" registration, the right window in addition to the "(default)" Outside the registry key to delete all, and finally delete the "Userchoice" registration;

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