Both tunes and documents can be downloaded from and uploaded to the site in a plain text format. If you are interested in this, here are the things you need to know about those formats.
The tunes are represented in ABC. This is a simple set of rules for representing a piece of music in plain text characters, using only the keys of an ordinary computer keyboard. Simple pieces can be represented extremely simply and easily, while suprising subtlety is available for more complex situations. In the world of using computers to deal with traditional tunes, it's emerged as a notation of choice, owing to its ease of use by both people and computers. There is a vast amount of material in this format to be found around the 'net, and a wide choice of software to support it in various ways and do useful things with it, and this website uses some of them to get the work done; it can render tunes as MIDI via abc2midi or typeset them as PDF, SVG or PNG images via either abcm2ps or abc2mtex. See the ABC references page for more details on this.
A document is a mixture of tunes and text, written according to rules I've made up. These can't be done as midi, but they can be rendered as PDF via abcm2ps, abc2mtex or LaTeX (which last can get you a page-numbered table of contents, if you're prepared to fiddle with it).