I am web designer and developer, i have worked online as web developer and designer on a website.
how could i prove my work experience to get pnp approved ???
and that's why usually such documents should be notarized, i.e., they should have a stamp certifying that the document was signed by the right person.
now, I don't think it's mandatory, but if you can afford it, it saves the immigration officer from making a phone call, and you get your application processed 5 minutes faster.
also, if possible, get the letter written in letterhead paper (with the name and address of the company), and if possible, a business card of the person signing it.
again, I don't think it's mandatory, but whatever you can include to make the IO's job easier, the better it is for you.