Because PDF format is more or less 'universal' - Almost all word processors can read documents in PDF format.
PDF is the format commonly used for email attachments because it is universal. It can be opened by Adobe Software, which most people have on their computer.
PDF Format
MIME
I believe you are referring to attachments? They are computer files which is sent along with an e-mail message. The file is not a separate message, but now it is almost universally sent as part of the message to which it is attached.
The most commonly used transparent video format in the industry today is WebM.
pictures
Yes, PNG is a picture format commonly used for images on the internet.
commonly used formats are png and jpg.
common attachment file types are .pdf (adobe reader), .doc (microsoft word), .jpg (jpeg image), and .zip (compressed files of any file format).
A compact tractor is a smaller version of an agricultural tractor and most commonly used attachments in compact tractors are front end loader and three point hitch.
There are many email programs available. Please see the folowing for information about commonly used email programs: linux.web.cern.ch/linux/scientific4/docs/rhel-rg-en.../ch-email.html
Getting directions and email.