I suggest you to disable the Adobe PDF add on using Group Policy instead of Registry setting.
When you set the group policy, it will create a proper registry key by itself.
You can refer steps below may help you to enable/ disable add on using group policy.
(1) Get the CLSID for the add-on you want to enable or disable:
A. Open IE, click Tools, and then click Manage Add-ons.
B. Double-click the add-on you want to change.
C. In the More Information dialog, click Copy and then click Close.
D. Open Notepad and paste the information for the add-on.
E. On the Manage Add-ons windows, click Close.
F. On the Internet Options dialog, click Close and then close IE.
(2) From the copied information, select and copy just the Class ID value.
Note:-
You want to copy the curly brackets as well as the CLSID:
{47833539-D0C5-4125-9FA8-0819E2EAAC93}.
(3) Open the Group Policy Management Editor and go to:
Computer Configuration\Policies\Administrative Templates\Windows
Components\Internet Explorer\Security Features\Add-on Management.
-OR-
Open the Local Group Policy Editor and go to:
User Configuration\Administrative Templates\Windows
Components\Internet Explorer\Security Features\Add-on Management.
(4) Open the Add-on List Group Policy Object, select Enabled, and then click Show.
The Show Contents dialog appears.
(5) In Value Name, paste the Class ID for your add-on, for example, {47833539-D0C5-4125-9FA8-0819E2EAAC93}.
(6) In Value, enter one of the following:
0= The add-on is disabled and your employees can’t change it.
1= The add-on is enabled and your employees can’t change it.
2= The add-on is enabled and your employees can change it.
(7) Close the Show Contents dialog.
Reference:
Using the CLSID and Administrative Templates to manage group policy objects
If you still want the solution using Registry key than you can refer steps below.
(1) Get the Class ID from IE > Tools > Manage Add-ons
(2) Create Registry key on location below.
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Ext\Settings{47833539-D0C5-4125-9FA8-0819E2EAAC93}]
“Flags”=dword:00000001
Note: 0 is for enable and 1 is for disable.
Reference:
Disable Particular IE add-on via registry