Why is Adobe Reader So Terrible?

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Mac OS’s ability quickly display and navigate PDFs shows just how awful a program Adobe Reader is. Adobe Reader is slow to load and reading documents is usually an arduous experience resulting in users who roll their eyes every time they realize they have to open a PDF file. I’ve never been a big fan of information in PDF format as opposed to XHTML, but why is using Adobe Reader such a horrible experience when Adobe controls both the format and the viewer?