PDF format itself is not intended to be edited. Normally you author your documents in a word processor (or in our case in HTML editor) and convert it to the “final” print-ready format: PDF.
There are some tools, which allow you to slightly modify PDF documents – for example, to substitute placeholders. But a full text editing of PDF with such tools would be rather a pain.
By default PD4ML does not protect PDFs from being edited. In order to disable that you would need to explicitly disable the editing permissions with setPermissions() API call.