Portable Document Format (PDF) file type is one of the most common file formats in use today. Its value comes from the fact PDFs always print the same way, and that PDFs are supposed to be read-only. This immutability can be assured by password protection and digital signing. Researchers working at the Ruhr University Bochum (Germany) have presented two possible attacks where the content of the PDF document can be altered by the receiver in such a way that the changes are undetectable.”]