Thank-you letters WYSIWYG vs html source content
You can turn off html encoding for CKeditor through UI
Navigate to Administer &gt;&gt; Customize Data and Screens &gt;&gt; Display preferences
Scroll down until you see Configure CkEditor button. Click on it to view all the settings. Under Advance options add entities option as False(as shown in image) and save the form.

Note: You will need to clear your browser cache to reflect the changes(and CiviCRM caches).
If you want to turn off the ck-editor completely for whole site than you can switch to text area. You need to navigate it to CiviCRM &gt;&gt; Administer &gt;&gt; Customize Data and Screens &gt;&gt; Display preferences. Change value for Wysiwig Editor from ckeditor to text area.
HTH
Pradeep