I'm in the process of writing a bi-lingual multi-column translation of an Adobe.pdf German book on some of the regional dialects in Germany. I have successfully tested .pdf pages that have test links to German and English YouTube websites for video illustrations of my subject matter.
One English language YouTube website video I have successfully tested in a .pdf test file is a video demonstration of examples of German regional dialects. Another video on the German YouTube is a longish (30 minutes) video about the post-WWII German "Kabineroller" small cars -- actually it is also a video 'travelogue' of sorts.
From my tests I can see that YouTube video links from within my text will work and will be highly useful -- when and if I can finish my Adobe .pdf eBook!
My tests were with external video sources, and for this reason my videos won't take up any space at all within my eBook. Obviously though, with my method if you want to use explanatory videos in your eBook those videos will have to be posted and available separately on some sort of a support website, i.e., a public or private website.