Hey Fred,
The PDF HTML and the online guide HTML are completely seperate. There isn't a way to change the html of a PDF, but you can modify the CSS through the PDF CSS Appearance setting.
The preview pdf button underneath the textarea where you input the custom PDF css is the best way to view the html structure and test your custom style.
If your guides aren't showing up in the input field of the preview form, make sure that your guides are published because private guides cannot be previewed.
Hopefully that answered your question. Let me know if you have any further questions!