PDF stands for Portable Data File and 4 indicates that each section of the code is made of 4 bars and stripes by 17 units. These can be between 3 and 90 rows.
This symbol is like stacked sets of 1D barcodes. Both numbers and letters can be encoded into these symbols. The symbol is made up of start and stop patterns, left and right row indicators and the data blocks in the middle.
This symbology is in the public domain meaning that anyone can make use of it without dealing with licing issues. If you need to generate these types of codes, you might want to check out the www.racoindustries.com web site.