HyperText Markup Language (HTML) is the format used to create documents on the internet including websites and emails.
HTML provides advanced formatting options not transferable to PDF including embedded videos or gifs. HTML can vary in display slightly depending on the browser or email client being used.
A HTML Form Letter setup in RMS will send as the email content and does not require a separate email body to be setup in RMS.
Portable Document Format (PDF) is a file format used for sending electronic documents in exactly the intended format.
PDF attachments cannot be altered except where Form Fields are used for permitting user interaction, their formatting, images, fonts and colours are contained within the document and not dependent on the Internet browser or email client being used to view.
PDF Form Letters setup in RMS send as an attachment to an email and require a separate email body to be setup.