Found this old article on Adobe Design Center. Good explaination of the different PDF/X variants.
Here’s what must be included in a PDF/X file:
- Fonts and images embedded.
- Page geometry (the trim box and bleed box) defined.
- Intended printing condition (output intent).
- Trapping key (Yes = True, No = False) defined.
- Title, creator, producer, creation, and modification dates.
Here’s what’s prohibited in PDF/X files:
- Live transparency, except in PDF/X-4 files.
- Layers, except in PDF/X-4 files.
- Encryption (security).
- Form fields.
- Interactive elements including movies, sounds, buttons, and hyperlinks.
- Annotations within the bleed box.
- Preseparated PDFs.
- Transfer functions.
- Embedded PostScript®.
PDF/X files do not set a minimum image resolution nor limit the plates used.
About the PDF/X standard and transparency
Originally released in 2000, PDF/X standards continue to evolve, mainly with the wide adoption of newer versions of the PDF specification. Here is the evolution of these standards and how they handle transparency in files:
- The PDF/X-1a:2001 and PDF/X-3:2002 standards are based on PDF 1.3 (Acrobat 4.0). Creating a PDF 1.3 file—such as from InDesign CS3, Illustrator CS3, or Photoshop CS3—flattens transparency.
- The 2003 versions of the PDF/X standard (PDF/X-1a:2003 and PDF/X-3:2003) are based on PDF 1.4 (Acrobat 5.0), but can also accommodate PDF 1.3 files. This standard does not support transparency. You can still use transparency in your design, but you must flatten the transparency before creating a PDF/X-1a or PDF/X-3 file.
- The PDF/X-4:2007 format saves PDF 1.6 or later files. This format preserves transparency (does not flatten it) and layers.