I am from Nepal and currently in Japan with Trainee Visa.

Salin, the required documents include your passport and its copy, the completed application form affixed with a passport-style photo, your valid entry permit in Japan, and an invitation letter from your friend. Inside the invitation letter, your friend should state that you will stay in his/her house. I think copy of email itinerary will be accepted.

And regarding the Invitation letter, as far as i know it will be emailed to me, so will it also be accepted? and are there any extra documents to send while sending Invitation letter?

Yes, I’m afraid the e-mailed copy cannot work. You can ask your friend to send the fax or scanned copy to you.
You can also ask your friend to send you the copy of his/her Chinese ID card or passport in case that it will be required.

I mean to say the invitation will be send to my email. So I'll print it out. Will it be ok?

Salin, usually, your friend needs to sign his/her name on the invitation letter. So if she/he sends the invitation by e-mail, there is no signature, so maybe it cannot work. Thus, a fax or scanned copy is better.