This phrase was coined by the Canadian philosopher, Marshall McLuhan. He proposed that people are not only affected by the content delivered over a medium, but also by the characteristics of the medium itself.
If you think about the reading experience of traditional formats like the PDF, you'll notice it's a very linear journey. To progress through the document, you have to scroll until you reach the end. There is virtually no choice available to the reader other than that.
Static scrolling documents dictate the reader's journey. Because of this, they fundamentally don't satisfy the autonomy aspect of self-determination. Your proposal feels like a lecture, like you have the power and they don't. This will always be a disadvantage when you want someone to make an intrinsically motivated decision.
By comparison, when you offer up your proposal in an interactive format (like what you're reading now), readers are given more choice and more freedom to engage on their own terms, satisfying their need to make independent decisions.