A format that fixes a document's appearance wherever it is opened. A named format rather than a concept.
An initialism coined by Adobe Systems in 1993 for a file format that fixes a document's appearance across every machine that opens it; portable from Latin portare (to carry), document from Latin documentum (a lesson, proof), format from Latin forma (shape).
A format that fixes how a document looks no matter where it is opened, keeping a document portable without letting its appearance drift.