You can now use the Forms app to create simple form letters.
A Letter is exported as a PDF file from a Form, and uses the form-specific values to replace placeholders in your custom template.
To customize the PDF layout for a form template, goto Configure Templates from the Left pane in the Forms web app, select a template and open the Export PDF tab.
In the Customize layout section, use the HTML editor to enter your custom letter text and layout. Click on Insert Field in the blue bar to select a Form Field value to be inserted dynamically in the PDF output.
To create a letter, open a form with this template, enter form values then click on Export PDF.
Note that this feature is meant to create letters with a simple layout (no tables or tabulations, no images, etc…)