At some point in our computing lives we will almost certainly stumble across a PDF (Portable Document File). These are extremely useful for transmitting a document, including any associated diagrams or pictures, in one single file which should look identical to the original on any compatible computer or mobile device. The fact that practically any modern day machine can use these files, coupled with the maintenance of the original appearance, has made this format extremely popular.
The possible applications are fairly obvious. For example, most modern day hardware and software doesn´t come with a printed manual but rather an electronic copy stored in PDF format on an accompanying CD. In the office, we send invoice copies in PDF format via e-mail as you can be sure that the recipient will view the document exactly as intended. Transmitting a complex document in an alternative format such as .doc would mean that the recipient would be unlikely to see things exactly as you intended, unless they had access to all the embedded pictures and fonts which are often unique to your particular machine.

بررسی ساختار امنیتی Postfix