These words are used to print XML preserving whitespace in text nodes:

write-xml ( xml -- )

xml>string ( xml -- string )

These words are used to prettyprint XML:

pprint-xml>string ( xml -- string )

pprint-xml ( xml -- )

Certain variables can be changed to manipulate prettyprinting:

sensitive-tags

indenter

All of these words operate on arbitrary pieces of XML: they can take as input XML documents, comments, tags, strings (text nodes), XML chunks, etc.