In today’s business world, PDF files are used for many different purposes. They can serve as proof of a variety of transactions and contracts, or they can function as the digital equivalent of paper documents. It is a widespread misconception, however, that PDF files are digitally unalterable and thus forgery-proof. Even PDF documents that have a digital signature can be forged. Scientists at the Ruhr University in Bochum, Germany, found that 21 of 22 of the most popular PDF readers did not recognize the manipulation of electronically signed PDF files (e.g. official letters, Amazon invoices, and legal texts).