A subset of the PDF file format that is used to guarantee document exchange without human intervention. The first version of PDF/X was introduced in 2001. See PDF and PDF/A. PDF/X-1 (2001-2003) RGB and LAB color spaces are not permitted. PDF/X-1a also prohibits OPI color separation and encryption. PDF/X-3 (2002-2003) Similar to PDF/X-1 except that RGB and LAB color spaces are permitted. Based on PDF Version 1.4. PDF/X-4 (2008) Supports transparency and optional content. PDF/X-5 (2008) PDF/X-5g supports OPI workflows. PDF/X-5pg supports external ICC Profiles, and PDF/X-5n adds more ICC Profile color spaces.