Acrobat and Reader warn you any time you try to open a URL (website) or a file attachment that uses a disallowed file type. URLs and disallowed file types are potentially dangerous because they can transfer or run programs, macros, or viruses that can damage your computer. Examples of disallowed file types include .exe and .zip files.
Documents can be secured in several ways. To view the security settings of a document, choose File > Properties in Acrobat or Reader. From the Document Properties dialog box, click the Security tab. If you have trouble opening a PDF, or you’re restricted from using certain features, contact the author of the PDF.
A password-protected PDF either requires a password to open or a password to change or remove restricted operations. If a document has restricted features, tools and options related to those features are dimmed.
Certifying a document assures recipients that the document is authentic. Certifying signatures also include information to allow or disallow particular actions, such as filling forms and modifying the document. When you submit a certified form, you are assured that it is going back to the valid author. A document with a valid certificate displays a Blue Ribbon icon . A PDF Portfolio with a valid certificate displays a Signature Badge.