I think more than a few of these projects are started and/or maintained by Germans due to the astonishing number of documents received - e.g., paperless-ng appears to have been done by a German, although neither the original Paperless nor Paperless NGX immediately appear to be.
I'm sorry to tell you that is a an oversimplification and especially for documenting expenses as a company/freelancer it's kind of worse.
Last time I checked if you want to follow the tax law to the word you're not allowed to change the medium:
If an invoice came as a paper copy (e.g. by snail mail), this paper copy is the original. If you scan it the digital version isn't.
If an invoice came as a digital document (e.g. a PDF by email), this digital document is the original - a printed version of that digital document isn't.
So if a tax inspector asks for "originals" it's technically almost impossible to provide them in the sense of the law. If even a tax inspector would care is another question.
And yes, they care about those rules and that you provide "originals" according to that definition - in particular that you didn't modify digital documents in any way. You can (and should) comply with that and there are service providers to help if you are to small to set that up yourself.
I admit that my last paragraph was kind of hyperbole, but I never heard (at least from other freelancers) of a tax inspector which wasn't happy with either everything printed or everything digital. I guess they really start to care if they suspect something fishy.