Not only do you have the option to setup criteria for generating documents automatically, users can also generate documents on demand using the WebMerge button on a Salesforce record. The objects automatically supported by WebMerge are: Account, Case, Contact, Lead, and Opportunity.
**If you are using another Standard/Custom Object, please read this article on setting up Custom Objects and Buttons.
From Setup, open the Object Manager and select the Object whose page you want to add the button to. From the left panel, select Page Layouts.
Drag the WebMerge button into the Custom Buttons section of the Page Layout.