As stated in the plugin description:
THE PDF TEMPLATE
Currently, the template for the each exported post is very basic (and a table, since floating doesn’t play nicely with the DOMPDF library), feel free to edit it here wp-content/plugins/simple-pdf-exporter/process/pdf_layout.php, only if you know what you are doing.
Your layout must be echoed in php. eg. echo ‘<div>the content goes here</div>’; or it won’t show up in the PDF.
You can use a custom CSS to customize the layout.
Create a pdf_export.css in your theme folder, otherwise the plugin’s default (and really basic) CSS will be used (you can find it here wp-content/plugins/simple-pdf-exporter/assets/pdf_export.css)
Some hundreds people use it, therefore, not so useless ??