If employer sent a signed offer letter in scanned PDF format to candidate via email, and candidate printed out the letter and signed his/her signature on it and resent back in scanned PDF to the employer, is the offer letter/contract a legal document in term of HR and legal perspective ? Can he/she tender resignation with a scanned PDF offer letter in which both employer and candidate has signed on it ?

If you don't want to stay, no one will make you. Unless you are a C-level position, I don't think a company would waste their time and resources to take legal action against you... Just turn down the offer nicely.

Since a necro tered, tell u a story

A university of mine got grad who accepted to do intern at Company Z
She put a lecturer as reference which the lecturer agreed to do so

Dunno wat happened, the grad didn't show up at the Company X on first day despite signed contract

Lastly, cases brought to court
The lecturer need to be summoned to the court

From that time, the lecturer never become reference for students again

That Company X put all effort money to sue the intern