Did you know that the Preview app in Mac OS X — that seemingly simple PDF viewer — can scan and add your printed signature to a document? A new addition to the Annotations bar makes it possible to take a photo of your signature using your Mac’s built-in iSight or FaceTime camera. Chris Rawson at TUAW describes how. follow link
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Capture Your Signature Using Preview
I think, this is a very useless feature. One can sign a pdf digitally in Acrobat or the free JSignpdf (http://jsignpdf.sourceforge.net/). Unfortunately Preview doesn't recognize these digital signatures.
Most human beings and organizations don't recognize digital signatures either. The link to JSignpdf for the few who may have the opportunity to use them is helpful.
If one uses Adobe Acrobat to sign a pdf, a related badge is added on the front page automatically. One also has different other options to display information about the certification. Furthermore, on opening a signed pdf Acrobat pops up a message about the digital signature. If one has the appropriate certificates installed, one can even verify the authenticity of the document.