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.
In Reader, you can type your name, draw your signature, or place an image of your signature on a document. You can also add text, such as your name, company, title, or the date. When your document is complete, the signature becomes part of the PDF.
(First time signing) In the Place Signature dialog box, choose how you want to place your signature:
Type my signature
Type your name in the Enter Your Name field. Reader creates a signature for you. You can choose from a small selection of signature styles. Click Change Signature Style to view a different style. When you are satisfied with your signature, click Accept.
Draw my signature
Draw your signature in the Draw Your Signature field. When you are satisfied with your signature, click Accept.
Use an image
Click Browse, and locate your signature file. When your signature appears in the dialog box, click Accept.
If you do not have an image of your signature, use black ink to sign your name on a clean blank sheet of paper. Photograph or scan your signature and transfer the image file (JPG, PNG, GIF, BMP, TIFF or PDF) to your computer. Don't crop the image, Reader only imports the signature.
You can send signed documents via fax or email using the EchoSign service. EchoSign stores a copy in your online account so you can easily access it later. You can log in to EchoSign with your Adobe ID or create an account.
You can get documents signed by others using EchoSign. EchoSign is an online service that lets users quickly sign documents in a web browser, without requiring a digital ID. The service keeps track of the entire process.
The EchoSign service emails your signers that you'd like them to sign a document. They review and sign the document on the secure EchoSign website. Once signed, both you and your signers receive the signed PDF in email. EchoSign stores the signed document in your account for future reference. For more information, visit www.echosign.com.
In Reader, only PDFs with Reader Usage Rights enabled can be signed using a digital ID. If you don’t have a digital ID, the onscreen instructions help you create one. For more information, see Digital IDs.
For adding a digital signature
Click Work with Certificates in the Sign pane, and then click Sign with Certificate. Draw a rectangle to place the signature. (If this option is disabled, the PDF creator did not enable the PDF for digital signatures.)
If you're viewing the PDF in a browser, in the toolbar at the top of the window, click the Sign icon 
.
In the Sign Document dialog box, do the following:
To view and sign the document in a static and secure state, use Preview Document Mode. Dynamic content such as multimedia and JavaScript are blocked. For more information, see Sign in Preview Document mode in Acrobat Help.
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:
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