If I remember correctly, the website of Erdinger states their 'Dunkle Hefe' needs to be finished within just 3 days after opening. I know it isn't pasteurized but I wonder, can we stretch that a little? 3 days for a keg is kind of short.... Why is that anyways? Is it because the yeast settles?
Where did you read this? I am unfamiliar with Dunkle Hefe exactly, but this is true for wooden casks. Typically a wooden cask is served with gravity or with a hand pump to lift from a cellar. There is an opening in the top that draws air in to equalise the pressure. This air will reduce the shelf life to 3 days. A "Donkey Pump" on the common sankey keg is similar. The beer will spoil quite fast.
This gels with what I've heard of Hefe's in general. The recommendation to store the keg upside down is definitely yeast-centric, when you flip the keg to tap it it'll stir the yeast up again... which you want.
The 3 day thing is a little more puzzling. Wheat beers in general are a style that should be had as fresh as possible, but 3 days is definitely pushing it. Most bars will try to finish a keg of a fresh wheat beer in 2 weeks or less, if memory serves.
From reading the Google automated translation of that PDF you're referencing, that sounds like a sales/marketing document. The way I'm taking the information is that the document is selection criteria for starting accounts with Erdinger to sell the beer at your bar. A lot of the document seems like quality control steps like making sure the draft system is setup correctly and properly balanced, making sure the lines get cleaned at appropriate intervals, and the 3 day thing for assuring max freshness.
From a consumer standpoint: It'll last more than 3 days, you can take however long you want if it's on CO2, but for best flavor for a wheat beer you probably want to drink it within a month or 2...probably a bit longer if it's been refrigerated.