Sign in to Acrobat Web at https://documentcloud.adobe.com with your Adobe ID and password, or with your social (Facebook or Google) account.
Use the Protect PDF service to add password security to a PDF. After the password is set, users must provide the password to view the PDF.
Steps to add password security to a PDF:
In the top navigation bar, click Share > Protect PDF.
Do one of the following to select the files that you want to protect and then click Continue:
In the Protect PDF dialog box, type the password in the Set Password field, and confirm the same in the respective field. For each keystroke, the password strength meter evaluates your password and indicates the password strength. Click Set Password.
Your PDF is now password protected. In the preview window, enter the password in the prompt to view the PDF.
A copy of the protected PDF file is stored in your Adobe Document Cloud account. The Recent section at the bottom area of the home page lists all the files you’ve secured recently. Alternatively, to see the list of all your files, click Documents in the top menu bar.