reason of sending unnecessary attachment with email ?

If you got an email with an unnecessary attachment to it, 99% out of 100 it would be a virus or a kind of virus like Spyware attached to it.

Even if you know that person from whom you got that mail. Because then it would be the case that the said person's email account have been hacked and the hacker has sent you this email.

What you should do is, if you know that person contact him and ask about it and if he/she don't know about it then ask him/her to change there password. And meanwhile delete that mail.

No technical reason. Mainly it is a commercial decision, to use branding, or make the source of mails easily recognizable at a glance (by showing the logo on top). Also it looks nicer when you print it. The main problem is when it is abused, and the mail gets really big just because of the attachment (but this shouldn´t be the case at 2 KB size).

Some people might think that adding the attachment will make emails more difficult to counterfeit (to avoid phishing). But this is not true, since someone who is seriously intending to do a fake mail, can easily replicate it by copying the pdf file from an original mail (and could actually make the matter worse, since the receiver may actually "trust" even more the fake message when he sees the pdf...).

you are working in MNC so it is possibility that some rogue hacker hack you account and damage your organization financially.don't open attachment without scanning this link.

otherwise it is a commercial technique to promote their organization.or any piggyback technique to check whether his email got to client or not.