Unlike a format like ePub a PDF document attempts not only to display the text but to preserve the look and exact placement of the information on the page. If the exact same font is not used then the display will not be exact. There is wide variation in the size and shape of letters in various fonts which will effect the look of the document and could, in fact, cause words to overlap. Thus it is important that the user have the same fonts as the original. The only way to guarantee this is to embed the fonts. Anything else leaves it to chance although certain fonts are so common that it is not a problem that appears often.