To sign a document in Acrobat Pro DC
Step 1:
Click on the Tools tab
Step 2:
Scroll down and click on the Certificates icon. Since you are already there, choose the Add button so it will add a shortcut to your S
Step 3:
Click on "Digitally Sign" and follow the same procedures below starting with step 2
To sign a document in Acrobat Pro XI and older
**These instructions assume you have an created a Digital Certificate within Acrobat Pro with the default appearance of Standard Text. If you do not, please work with ITO support before attempting to digitally sign PDF documents if you are unable to set up a Digital Certificate from within Acrobat Pro.**
Step 1:
In the top, right side of the Toolbar, click on "Sign".
From the Sign menu, click "Place Signature"
The mouse indicator will change from an arrow to a target/plus sign.
Step 2:
Locate an area on the PDF document where you can place your digital signature. Click, hold, and drag mouse to place signature, then release.
Step 3:
When released, the "Sign Document" dialogue box will appear.
If this is the first time signing a document, you will need to create a new APPEARANCE. Select your certificate identity under Sign As, then set the appearance. You have to choose the Identity that shows your name and with The University of Texas at Austin RCA as the Certificate Issuer. I always import a handwritten signature I've scanned and combine them into one appearance so they can serve dual purposes in case they need printing. You could even uncheck all the text boxes and leave just your name if you wanted to, or if you scanned a signature and imported the graphic, you could set the appearance to only show your handwritten signature if you wanted to. For the free version of Acrobat Reader DC, the handwritten scan has to be saved as or exported as a PDF. Be sure to crop the handwritten signature so that no adjustments will be necessary upon signature placement.
If no one else needs to sign the document – click Lock Document After Signing
If the PDF requires multiple digital signature – do not click on this
WARNING!: THE FIRST TIME YOU SIGN A DOCUMENT ON A MAC, IT MAY ASK FOR YOUR LOGIN KEYCHAIN PASSWORD. THIS SHOULD BE YOUR EID PASSWORD. YOU'LL WANT TO ENTER YOUR PASSWORD AND CHOOSE ALWAYS ALLOW.
Step 4:
Click "Sign" and…
…a "Save As" dialogue box will appear. 
The document will need to be saved with each signature obtained. Best Practice: save the PDF document to a folder you set aside for Signed Documents (on desktop, in Box, etc.).
Step 5:
In the top, left side of the toolbar, you should see "Signed and all signatures are valid" with a green checkmark. If you do not, you did not download the latest Trusted Root Certificates validation. 
Step 6:
Attach the PDF document to an email and send to next signer or final destination.
Additional info:
If you want to change the appearance of you signature, you can choose "Create New Appearance" and work with the preferences of your operating system. Contact ITO support for assistance or click on green question marks for Acrobat Help. 
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