If you want to create a document that contains checkbox images rather than “True” or “False” values, you can do so in two different ways.

Display Checkboxes With The Checkbox Merge Field Attribute

The easiest way is to add the checkbox merge field attribute to any boolean field that you want to display as a checked or unchecked checkbox.

Example Merge Field:

{{!Opportunity.Checkbox__c}}

Standard Checkboxes:

Display a standard checkbox (dotted white border with black check) that is checked if the boolean field is true, and unchecked if it is false:

{{!Opportunity.Checkbox__c checkbox="true"}}

Display a standard checkbox (dotted white border with black check) that is unchecked if the boolean field is true, and checked if it is false:

{{!Opportunity.Checkbox__c reverse_checkbox="true"}}

Black Checkboxes:

Display a black checkbox (solid black border that fills in black with a white checkbox) that is checked if the boolean field is true, and unchecked if it is false:

{{!Opportunity.Checkbox__c checkbox="black"}}

Display a black checkbox (solid black border that fills in black with a white checkbox) that is unchecked if the boolean field is true, and checked if it is false:

{{!Opportunity.Checkbox__c reverse_checkbox="black"}}

Radio Buttons

Display a radio button that is checked if the boolean field is true, and unchecked if it is false:

{{!Opportunity.Checkbox__c checkbox="radio"}}

Display a radio button that is unchecked if the boolean field is true, and checked if it is false:

{{!Opportunity.Checkbox__c reverse_checkbox="radio"}}

Learn more about merge field attributes here.

Display Checkboxes With CSS

Checkboxes can also be displayed using CSS styling similar to the following example:

<style type="text/css">.checkbox {
   border: 1px solid black;
     padding-left: 2px;
     padding-bottom: 2px;
   height: 9px;
   width: 9px;
 }
</style>
CHECKBOX
   <div class="checkbox"> </div>

Display Checkboxes With Formula Fields

You can also display checkboxes by leveraging a formula field.

By adding the following formula field to your object, you can easily leverage it to place checkbox images into your S-Docs. (You do not need to add the field to the page layout.)

Note: Your formula field should have a Formula Return Type of Text and should be referenced as rich-text when merging into a template. The field used in the example, IsAccount__c, should be a checkbox field on your object that is checked by default.
IMAGE(
    IF(IsAccount__c, '/img/checkbox_checked.gif', '/img/checkbox_unchecked.gif'),
    'CheckBox'
)