Their failing is in relying upon the browser component included with their compile. There may be others, but I am aware of only one browser available to programmers. While I can't speak for others, the browser we used is limited in capability, and may behave unpredictably. For example, it can not handle some graphic designs, table content, and so forth. And they will likely remain so.
Numbers are debatable, and differing points of view abound.
While this percentage can be expected to decline, it won't happen overnight. If you compile only for Windows, you eliminate all MAC users. Since a free Acrobat reader is available on most systems, you can reach nearly all computer users with this format. I disagree for two reasons. A lot of people do not like this reader, are not comfortable with it, and find it difficult to use.