WHAT IS PDF/X?
The PDF/X file format was created with the objective of eliminating problems when making PDF files for press production. It is not a substitute for PDF but rather a "flavor" of it customized for press.
The reason to create the PDF/X format was that not all PDF files are suitable for press, which is one of the strengths of PDF format, after all. Distiller solved that problem partially by enabling users to save and share .joboptions files, but in order for them to work, a user needs to have Acrobat. Because these days there are several third-party programs that do the same task as Distillercreate PDF files from PostScript filesusing .joboptions files with them is impossible. This prompted many printing industry leader companies to work on a standard format that could be used with press production in mind, and that's how PDF/X was created.
The difference between those two working profiles is mainly that of different dot gain values. Traditionally, in the United States, printers have used negative film to produce printing plates, but in Europe positive film is the standard. The result of these different methods is that a halftone done in the United States has a larger dot than the same one done in Europe, resulting in a darker U.S. print. To ensure accurate reproduction, the SWOP and Euroscale profiles compensate for that variation.
Currently there are two main versions of PDF/X, each suitable for a distinct market. Which to use? It mainly comes down to where you are. If you are in North America the PDF/X-1a format is right for you because it uses the U.S. Web Coated (SWOP) working profile. If you are in Europe, the PDF/X-3 is for you because it uses the Euroscale Coated profile instead.
In addition to those two, the PDF/XA standard is currently being developed as a format for optimum archival of documents.