Fax is short for facsimile; the idea is that modern computing
and telephone technology allows you to scan a document into a machine,
send a reasonable facsimile of that document over the phone, and have a
printer at the other end of the phone line translate the data and print that
facsimile for the benefit of the recipient. This has the advantage of sending
something close to an actual document over long distances in real time.

I know university people who insist on pdf files (documents whose file name
ends in .pdf, readable by Adobe's Acrobat Reader software) of journal
articles because only pdfs constitute reasonable facsimiles of the actual
printed journal or the articles contained in it. If wonder if DocJohn uses pdf
files of journal articles as part of his job when physical copies of a given
journal are out of reach for him.