PDF417 is a stacked barcode: rows of linear-looking codewords stacked on top of each other, read as one symbol. It holds well over a thousand characters and, like a QR code, carries error correction that lets it survive damage or a partial obstruction.
Its signature use is travel and identity. Airline boarding passes, many event tickets and most driving licences in North America are PDF417, because the format can hold a full record, not a pointer to one, and because the installed base of scanners at gates and checkpoints is built around it.
The shape is wide and short rather than square, which suits a strip across a pass or a document but takes more area than a QR code for the same payload. Where the scanner reads both, a QR code is usually the more compact choice.
Both Apple Wallet and Google Wallet render PDF417 on passes, so a program moving airline or ticketing barcodes into a wallet can keep the exact symbology the existing gates expect. QR and barcode identification covers matching the format to the hardware already in place.