If you want your content to contain lots of images and video (and you should), doing so on the PDF will dramatically increase the file size. And contrary to popular belief, size DOES matter.
Every extra moment between your customers and your content is another chance for them to lose interest and click onto something else.
With the PDF, everything in the document has to be downloaded at once, whereas with web browsers, content can be downloaded progressively, making it a quicker experience for the reader.
The only real way to speed up PDF download time is to reduce the quality of your media, but then you’re just trading off one negative point for another.