For a full list of articles about security, see Overview of security in Acrobat and PDF content.
Acrobat and Acrobat 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, press Ctrl + D (Windows) or Cmd + D (macOS) in Acrobat or Acrobat Reader. From the Document Properties dialog box, select the Security tab. If you have trouble opening a PDF or you’re restricted from using certain features, contact the author of the PDF.
Password protected
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.
Certified
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.
Adobe LiveCycleRights Management ES configured
Adobe LiveCycleRights Management ES (ALCRMS) lets you manage accounts and configure security policies for your organization. ALCRMS lets you apply a policy to documents without the need for digital signatures and certificates.