The PDF417 barcode symbology is a versatile and widely used two-dimensional barcode system, known for its ability to store large amounts of data securely and compactly. Invented by Dr. Ynjiun P. Wang in 1991, the PDF417, which stands for Portable Data File with patterns that are 17 units long, has become a standard in various industries for its robustness and flexibility. The symbology can encode text, numbers, files, and actual data bytes, making it suitable for applications such as transport, identification cards, and inventory management.

One of the key features of PDF417 is its error correction capability, which uses Reed Solomon error correction instead of check digits. This allows the barcode to remain readable even if part of the symbol is damaged, with error correction levels that can be adjusted depending on the amount of data and the likelihood of damage. The structure of a PDF417 barcode is composed of multiple rows, each resembling a small linear barcode, stacked on top of each other. This stacking enables the encoding of a significant amount of information in a relatively small space.

The PDF417 barcode is customizable, allowing users to specify dimensions for the narrowest vertical bar (X dimension) and the height of the rows (Y dimension). This flexibility ensures that the barcode can be tailored to fit different printing specifications and scanning environments. The symbol itself consists of 3 to 90 rows, each with a start pattern, data codewords, and a stop pattern, ensuring consistency and readability across various platforms and devices.

In terms of data density, the PDF417 can encode up to 1,850 text characters or 2,710 numerical digits, providing a substantial amount of information in a single barcode. This high-density encoding is achieved through a system of codewords, with each codeword representing one of 929 possible values from one of three different clusters. The scanner reads these codewords and, due to their unique clustering, can determine the row from which each codeword originates.

The PDF417's public domain status further contributes to its popularity, as it allows for widespread implementation without the need for licensing fees or restrictions. This openness has facilitated the adoption of PDF417 in governmental applications, such as the RealID compliant driver licenses in the United States and border crossing cards issued by the State of Israel.

Moreover, the PDF417 symbology supports linking, where symbols can connect to other symbols scanned in sequence, thereby expanding the total amount of data that can be stored. This feature is particularly useful in document management and tracking systems where a single document may need to reference multiple barcodes.

Overall, the PDF417 barcode symbology represents a powerful tool for data storage and retrieval, offering high capacity, flexibility, and resilience against damage. Its continued use in various sectors underscores its reliability and the critical role it plays in modern data management and security protocols. Whether for commercial or governmental use, the PDF417 remains a cornerstone of contemporary barcode technology.