A major benefit of PDF over other document formats is its capacity for controlling access to content and the way it can be modified by users. As there are many levels of security that can apply to a document, the document author needs to decide which measures are appropriate for the document being created.
The intended use of the document is the key factor here. For instance, if you have a press release in PDF, then you don't want to restrict who can open the document, because you would like as many people as possible to read it. But in that case, you probably won't want people to be able to alter its contents. Using security properties lets you control things like:
A digital signature is similar to an analog signature, in that it indicates approval of the document contents at the time of signing. The difference is that while an analog signature is represented by a physical mark, a digital signature is represented by a set of digital information unique to you. If a document is changed after a signature is applied, then it is no longer the same as the document that was approved. Hence, the signature will become invalidated.
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