PDF/X is a collection of standards defining a number of conformance levels, all of them targeted at ensuring predictable and consistent printing in a professional print environment:
All of these standards are published as parts of ISO 15930, under the general title Graphic technology — Prepress digital data exchange using PDF:
Part 1: Complete exchange using CMYK data (PDF/X‑1 and PDF/X‑1a);
Part 3: Complete exchange suitable for colour-managed workflows (PDF/X‑3);
Part 4: Complete exchange of CMYK and spot colour printing data using PDF 1.4 (PDF/X‑1a);
Part 5: Partial exchange of printing data using PDF 1.4 (PDF/X‑2);
Part 6: Complete exchange of printing data suitable for colour-managed workflows using PDF 1.4 (PDF/X‑3);
Part 7: Complete exchange of printing data (PDF/X-4) and partial exchange of printing data with external profile reference (PDF/X-4p) using PDF 1.6;
Part 8: Partial exchange of printing data using PDF 1.6 (PDF/X‑5).
All of the formal PDF/X development work is done in ISO TC130/WG2/TF2, with significant historical support from CGATS SC6/TF1 and from the bvdm and ECI. Contact Mary Abbott at NPES (firstname.lastname@example.org) for more information or to get involved.
Informal development discussions are held on a mailing list accessible at http://finance.groups.yahoo.com/group/pdfx_revision/
A number of the normative references for various PDF/X conformance levels can be obtained from the NPES standards workroom at http://www.npes.org/standards/workroom.html
A PDF/X FAQ may be obtained from http://www.globalgraphics.com/products/pdfx/index.html