Checkmark Stamps.
Adobe Reader XI -
Open the PDF. Click "Comment" (upper right of Reader).
Click "Annotations" to display the Annotations tools.
(the Stamp tool lives here)
Click the Stamp tool (top row - far right icon - the shape ids it).
Click "Sign Here" and select the check mark.
You can also make use of the Sticky Note.
While the default for this is a "bubble", once placed you open its properties and change the icon to a check mark (change to desired color also). To "lock" this configuration tick the "Make Properties Default" in the Sticky Note Properties dialog.
Be well...