Reader lets you sign PDFs and incorporate that signature into the file. If you are viewing a PDF on the web, download the PDF first before you sign it.
Some documents have security applied that prohibits electronic signatures. Print such documents and then sign the printed copy.
You can get documents signed by others using the Send for Signature tool, which relies on cloud services provided by Adobe Sign. It lets signers quickly sign agreements from anywhere using a web browser or mobile device. Signers are not required to sign up or purchase any Adobe product to sign the agreements.
The Send for Signature tool also helps you track the entire signing process. The tool sends an email to your signers asking them to sign an agreement. Once signed, both you and your signers receive the signed PDF by email.
The creator of a PDF can encrypt the document with a certificate for verifying digital signatures. Certificates that you receive from others are stored in a list of trusted identities. This list resembles an address book and enables you to validate the signatures of these users on any documents you receive from them.
For more information, see these articles in Acrobat Help: