Applying to study abroad requires professors to mail paper letters of recommendation. In this case, should the letters of recommendation be handwritten or written on a computer, printed and then signe

It is best to print it out on the school's letterhead and have it signed by the recommending professor. If sent electronically, it can be scanned into a pdf file. Paper letters can be sealed in the school's official envelope and mailed together with your other application documents.

If it is a paper document, it is best to print on 80g A4 paper.