Editable vs Non-editable Output in AutoMailMerge
Make Output Forms Non-Editable
The mail merge can produce two different kinds of output PDF documents – interactive PDF forms or flattened regular PDF files. This is controlled by the a single checkbox in the mail merge settings. If you choose an option to flatten output PDF forms then all interactive page elements such as form fields, links, comments, buttons will be converted into a regular text and graphics. The output forms fields will be non-editable.
Flattening output PDF forms
Making Part of the PDF Form Non-Editable
It is often necessary to keep part of the form fields editable and leave the rest interactive to allow recipient to enter new information or edit the existing one. This can be accomplished by turning "Flatten output PDF documents" checkbox OFF and then editing properties of the form fields in the input PDF form to make the fields "read-only". Use the Prepare Form tool that comes with Adobe Acrobat to edit fields.