Should I sign a motivation letter? I'm planning to apply for a PhD program and I wonder whether I should sign the motivation letter or not. I have read many articles about the subject, and many stated that if I'm going to send it online it's not necessary to sign it, but if I'm going to send it as a hard copy I should sign it. 

I'm kind of confused because the university I'm applying for wants me to send it online and also as a hard copy.
In any field I've heard of, as well as outside academia, if you are sending a digital copy (pdf, email, doc) you do not need to sign anything unless specifically requested. If you have to mail a copy of it is commonly customary to sign it, but no one will likely care either way. Good luck in your applications!