PDF417 is a high density 2 dimensional barcode symbology that essentially consists of a stacked set of smaller barcodes. The symbology is capable of encoding the entire (255 character) ASCII set. PDF stands for "Portable Data File" because it can encode as many as 2725 data characters in a single barcode. The complete specification for PDF417 provides many encoding options including data compaction options, error detection and correction options, and variable size and aspect ratio symbols. The symbology was published by Symbol Technologies, Inc. to fulfill the need for higher density barcodes. The low level structure of a PDF417 symbol consists of an array of code words (small bar and space patterns) that are grouped together and stacked on top of each other to produce the complete printed symbol. An individual code word consists of a bar and space pattern 17 modules wide. The user may specify the module width, the module height, and the overall aspect ratio (overall height to width ratio) for the complete symbol. A complete PDF417 symbol consists of at least 3 rows of up to 30 code words and may contain up to 90 code word rows per symbol with a maximum of 928 code words per symbol.
The code words in a PDF417 symbol are generated using one of three data compaction modes currently defined in the symbology specifications. This allows more than one character to be encoded into a single data code word. Because different data compaction algorithms may be used, it is possible for different printed symbols to be created from the same input data. The symbology also allows for varying degrees of data security or error correction and detection. Nine different error correction levels are available with each higher level adding additional overhead to the printed symbol.