What is the differences between purchasing Harvard Business Review PDF and reading the article?
Many publisher offer a teaser - an excerpt of an article - as an advert for the full article. The teaser may contain the abstract, some graphics, maybe a lay summary or press release about the article, and so on.

In the case of your Harvard Business Review article, they let you read the first eight paragraphs, which gives you an idea of the article's content and style. The full article is about five times longer than that excerpt.

If you buy the pdf, you get the full article, together with its references, and any supplementary information. (though in this particular case, there are neither references nor supplementary information).

Note that five pages of text can get turned into nine pages within a journal, thanks to graphics and other materials that appear in the PDF, but don't appear in a text-only version: for example, in the PDF of the article in question, the first page consists of a single image.  The text-only version may or may not contain the side-bars that appear in the PDF ("Things to Look Out for When Judging Expertise").