A PDF file can easily be locked, making it almost impossible to edit, (sometimes "A-PDF Restrictions Remover" can help.), but we know nothing about your file - apart that it apparently not really is your file. In general, a PDF file should never come with special fonts unless they are embedded. If you are lucky, chances are that it is enough to mark all the text (page by page) and change the font into universal fonts like Arial or Times New Roman, or simply convert the file from PDF into RTF or Word format, and then edit.
But you better begin with installing the missing font.