What is a PDF/X-1a, PDF/X-1b, PDF/X-3?
How does a PDF differ from a PDF X file?
The average PDF can have anything in it, including things that cause the PDF file to be usable from the view or prepress production. A PDF X file limits the scope of the PDF and what is in it and makes it something that can be used in prepress production without worrying whether the file was prepared correctly. It is simply a PDF file with restrictions.
It can get a little confusing because there are at least three different variations of PDF X. The variations are PDF X-1, PDF X-2 and PDF X-3. The PDF X-1 standard was originally published in 1999, and updated in 2001. This is the original exchange standard and is based on PDF 1.3.
The standard defines two specifications: PDF X-1 and PDF X-1A. PDF X-1 is the original standard. PDF X-1A is similar to PDF X-1 except that it prohibits OPI (image linking and replacement) and makes sure the images are included in the PDF file.
PDF X-3 is based on PDF X-1A, and is based on PDF 1.4. With PDF X-3, color-managed data may be included. (In PDF X-1A all colors are forced to CMYK, with spot colors being optional, and no color management is allowed.) In a PDF X-3 file, there may be data that is color-managed using embedded ICC profiles. PDF X-3 is expected to be approved this spring. RGB data is allowed as long as it is in a device-independent space. This may be helpful for certain workflows, but allows too many color "unknowns" for most commercial printing operations.
PDF X-2 is under development. PDF X-2 is based on PDF X-3 and allows color-managed data to be included, as well as OPI and non-embedded fonts, as well as tools to allow identifying these links to fonts and images.
What is the difference?
PDF X-1 and PDF X-1A pretty much lock down the file, making sure that all the fonts, images, and color space are restricted to a narrow set of choices that tend to work well in printing environments. PDF X-3 and especially PDF X-2 provide more flexibility with PDF X-3 locking the file but allowing more options with regard to color management, and PDF X-2 allowing more options with regard to color as well as image and fonts linking and other custom workflow specifications. PDF X-1 will be most common when you want to lock the file down and make it bulletproof. PDF X-3 and PDF X-2 will be used for customized workflows in which the PDF creator and the printer are working closely together and want to take advantage of some of the benefits offered by having a more flexible but complex workflow.