Adobe PDF documents have security settings which can, among other things, prevent anyone from modifying the document. This is helpful, for example, when sending electronic copies of contracts for review. But it can also be a pain when the document needs to be changed but the password has been forgotten or the person who knows the password is no longer working for the company. The linked article has a solution that works in Windows systems: print the PDF to an XPS file (the XPS “printer” is automatically supplied by Microsoft), then open the XPS file and print it to a PDF file. This will give you an identical copy of the original document, but without any security restrictions.