In many countries, the income of companies is public and accessible. My question is whether this is correct also for Germany, and where I could look up such information?
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This is also true for Germany. There's an official service by the Bundesministeriums der Justiz to search for all important information required to be published about companies:
Documents are mostly not free of charge but from what I discovered much cheaper than "2nd-hand unofficial websites" that offering this service as well. With a credit card you can directly download the requested documents on their page as PDFs without even registering.you can use sites as http://handelsregister-online.net/ (a report costs usually arround 15 EUR) but most reports are up to 2 years old.