The PDF/X format is a subset of PDF
used for pre-print processing of electronic documents. It is certified by the International Organization for Standardization as Standard ISO 15930-1:2001. As set forth in the above standard, PDF/X may be used for a blind interchange of electronic documents whose printing output intent is fully defined and does not require additional negotiation between sender and recipient.
The PDF format was originally designed as an electronic representation of any completed printed product but not for pre-print processing. However, given the breadth of its spread, it came increasingly to be used in this application. Thus, in time PDF/X was created. At this time, it comes in three versions: PDF/X-1, PDF/X-2, and PDF/X-3.
PDF/X-1 is the very first PDF/X format. It, in turn, comes in three versions: PDF/X-1:1999, PDF/X-1:2001, and PDF/X-1a:2003 (it was derived from PDF V1.4). The latest of the three was the version to become certified as a standard for pre-print document processing.
The PDF/X-1a specifications are very clear. They explicitly prohibit the use of annotations, bookmarks, comments, indices, multimedia embedding, hypertext links, and RGB images. In addition, for compliance with the standard, all fonts have to be embedded into the electronic document and colors have to be defined as either CMYK or spot.
As time went by, the PDF/X-1a capability began to fall short of all the functionality of modern printing. As a result, PDF/X2 was developed, a format with much more lax requirements. It allows the use of ICC-profiles, OPI specifications, and includes RGB support. This format is obviously much more feature-rich. Probably, too rich for many of its applications which makes things more complicated and increases the risk of error. As a result, PDF/X-3 was created as a "middle ground" between PDF/X-1a and PDF/X-2. It is most similar to the former but implements support for Lab and RGB with profiles.
See also: PDF/A