The PDF format was designed for portability, with the word “portable” in its very title. But whereas in the past documents were physically faxed or reprinted, the internet has simplified the process of sending and copying. However, with this ease comes the need for greater security measures designed to prevent any harmful, or even inadvertent manipulation of the original document.
In PDF Plus you can use passwords to restrict users from opening, printing, and editing PDFs. You can digitally sign or secure PDFs with certificates to protect the integrity, validity and security of your documents (For an overview of the different types of security, see Viewing PDF Security Settings).
When having trouble opening or performing certain functions with a secured PDF, it is best to contact the original author or the person you received the document from.