As a company, Dunder Mifflin isn't exactly a successful business. The company was unable to adapt to an increasingly paperless world, and its attempt to revitalize the company backfired when they hired Ryan Howard (B.J. Novak) for an executive position.
Ryan subsequently got arrested and fired for committing fraud by inflating sales numbers and misleading shareholders. And the company also had limited ethics, as can be seen when Michael Scott (Steve Carell) is directed to steal the clients from a friendly, family-owned competitor
Dunder Mifflin came under the management of the horribly mismanaged Saber, which produced faulty printers and triangle-shaped tablets. By the end of "The Office," former Dunder Mifflin executive David Wallace buys the majority share in the company, reverting it back to a somewhat normal operating procedure.
Of course, Dunder Mifflin is not much more than a reason for "The Office" to take place, but that's more than enough to make it one of the best fictional businesses of all time.