The PDF/X-4 standard

This version of the PDF/X standards is the major change since the release of PDF/X-1a. It represents the maturity of PDF/X, with full acceptance of device independent colours and transparencies. It also includes the use of optional content (layers), Open Type fonts and some other details that make it much more structurally sound than its predecessors were, which it should replace (unless we have a very solid reason not to do so).

This standard was published in 2008 and it was revised in 2010. Therefore, it has two revisions:

• "PDF/X-4:2008"
• "PDF/X-4:2010".

For practical purposes, there are few differences between them, so we will use the 2010 version whenever possible.

Besides, there is a variant called PDF/X-4p, which is explained at the bottom of this page.

Prohibitions and obligations

The prohibitions and obligations are what define a PDF/X level. These are the main ones in PDF/X-4. Some of the prohibitions are due to the fact that the features in question did not yet exist in PDF format when the standard was defined.

• PDF format level: The use of PDF format level 1.6 is mandatory. Other levels are not supported.

  The use of features not included in the PDF definitions of Level 1.6 (e.g. third-party extensions) is forbidden.

  The use of "alternate presentations" is explicitly prohibited.

  Document larger than 2Gb are allowed.

• Security: No access restrictions (passwords of any kind, etc.) are allowed. No encryption may be used.

• Self-contained: All elements used must be inside the document. The use of external linked elements (images and fonts) and the use of OPI is not allowed.

  An exception to this rule is the PDF/X-4p variant, explained at the end of this page, which refers to the use of external colour profiles.

  Embedding of attached files is allowed.

• Metadata: Every document must have the following metadata: Creation date, modification date and Document title. This metadata must go in a general set of metadata in XMP format, it is no longer mandatory that they are included in the "Info" dictionary (as in previous versions of PDF/X). But if they do, discrepancies between the two sets of metadata are forbidden.

  Besides, the document must incorporate the internal tags ("GTS_PDFXVersion" and "GTS_PDFXConformance"), which identify the level and revision of the PDF/X standard for which the document was created; a "Trapped" trapping key indicating whether the document has trapping applied or not (the value "unknown", allowed outside the PDF/X standards, is not allowed); and, in addition, every document must carry a unique identification (document ID).

  The "PDFXVersion" key must specifically use PDFDocEncoding (not the Unicode one).

• Interactivity: The presence of interactive elements (JavaScript, active forms, Flash, etc.) and multimedia (videos, sound, 3D, etc.) is forbidden.

  The use of non-printable comments within the printable area is forbidden.

  The use of XFA (Extensible Forms Architecture) is not allowed.

• Optional content (layers): The use of PDF layers (called "Optional Content Groups": OCG) is allowed (this is the first level where it is allowed).

• Fonts: All fonts used must be fully or partially embedded as subsets (at least all characters used).

  Embedding of OpenType fonts is allowed. These fonts are not passed as is to the PDF, with all their qualities, but only partially. In the resulting PDF, OpenType fonts are identified, depending on their origin, as ‘"Type 1 Composite Font" or as "TrueType Composite Font".

  The internal name of a font must be UTF-8 encoded.

  The explicit use of the .notdef glyph as a character (which might appear if a character is missing, which in turn would be a bug) is forbidden.

• Transparencies: Blending modes such as multiply, screen, overlay, etc. are allowed. It is not necessary to flatten the content. The transparencies supported are only those described in PDF level 1.6. Pixel mask (softmasks) are also supported.

• Colours and inks: In addition to the device dependent colour modes and classic spot colours already supported in the previous levels, the use of device independent colours with the use of ICC and Lab colour profiles is fully supported (the functionality is more comprehensive than in the PDF/X-3 level).

  The use of ICC level 4 profiles is allowed.

  Vector gradients (smooth shading) and patterns also support these colour modes.

  The use of the NChannel colour spaces is supported.

  A precise description of all colorants forming a DeviceN or NChannel colour space is required. In addition, the description of dot gains in the description of DeviceN or NChannel colours is prohibited.

  If spot colours are used, their alternative colour space values must be the same in all their instances.

  The alternative colour space of spot colours can be defined in Lab mode (besides DeviceCMYK and DeviceGray).

  The use of a rendering intent not one of the four classical rendering intents (ISO recognised) is explicitly prohibited.

• Compression: Compression can be used with some algorithms only: JPEG, JBIG2, JPEG2000 , ZIP, CCITT (type 3 and 4), Run-Length (RLE), and ASCII85. LZW is prohibited.

  Compression of stream objects is allowed.

  JPEG2000 compression is allowed (although some minor colour-related details are forbidden).

• Images: Pixel objects with a 16 bit depth are allowed.

  Alternate images are allowed, but they cannot be the default version for printing.

• Pages and dimensions: Page geometry (page boxes) must be defined using the MediaBox, and TrimBox or ArtBox. The BleedBox is optional in case of bleeded printing.

  Page boxes must be correctly nested.

  The simultaneous presence of TrimBox and ArtBox page boxes is forbidden. Only one of them can be used.

  Different page sizes are not allowed in a multi-page document (each page box size remain the same size through the whole document).

• Prepress: Preseparated pages (where colour separation has already been carried out and each page represents a plate) are forbidden.

  Embedding PostScript code and transfer functions is nnot allowed.

  The use of halftones other than type 1 or 5 (out of the ten defined in the PostScript specifications) is forbidden and some minor details related to halftones too.

  The use of scaling factors is forbidden.

• Printing system: It is mandatory to include information about the printing system for which the document has been created. This must be done through a structure called "output intent", which is included in the "catalog" dictionary (and explained in more detail in another page) and it is the only place where this information is provided.

  To avoid confusion, it is forbidden for a document to have more than one output intent (which the PDF format does allow at this level though).

  It is mandatory that the output intent be made via a colour profile and that this profile be contained within the document (except in the case of PDF/X-4p variants, see below). Simultaneous reference to standardised printing conditions, such as "FOGRA27" and a colour register (such as www.colour.org) are allowed, but this reference is no longer required, as the colour profile is included.
  
  As device independent colours are allowed, in this level of PDF/X the output intent is more important than in PDF/X-1a.

  Colour restrictions in the output intent: The possibility of using uncalibrated and device independent colour elements together in the same document imposes some restrictions on the output intent:

  • If DeviceCMYK is used in the document, the output intent's profile must be a CMYK one (it cannot be either RGB or greyscale).
  • If a document contains DeviceRGB elements, it cannot contain any other colour mode and the output intent must be an RGB profile (of type "ptr", not "display").
  • If there are greyscale elements in DeviceGray colour mode, the output intent's profile must be greyscale or CMYK.
  • If calibrated or device independent colours (Lab, CalGray or and ICC colour profile) are used, there is no restriction for the profile of the output intent, which can be greyscale, CMYK or RGB.
  • Objects defined in CMYK whose colour profile matches the profile of the output intent (which will consequently be CMYK) must be defined as DeviceCMYK and cannot incorporate that colour profile.
  • Any transparency blend space must be a CMYK profile if the output intent is a CMYK profile.
  • The use of colour modes other than Greyscale, RGB or CMYK for output intents is expressly forbidden.

  • If spot colours are used in separation or DeviceN colour modes, this imposes no restrictions on the output intent's colour profile, which can be grayscale, CMYK or RGB.

    The exception is that if classic spot colours are used (i.e. with alternative CMYK device colour space), the output intent's profile must be CMYK (a standardised printing condition can be referred to as well). This restriction includes DeviceN colour spaces.

  • If the alternative colour space definition of the spot colours is DeviceGray, the output intent must be  a greyscale profile.
  • If the alternative colour space of a spot colour is device independent (Lab), the output intent colour profile must be grayscale, RGB or CMYK (printer kind ("ptr"), noy display).

The 'P' variant: PDF/X-4p

From Level PDF/X-4 onwards, a sub-variant with the suffix "p" (the p stands for "partial") has been defined for each level. It is intended for closed workflows, that is: Those which do not receive materials from outside and where the components are always the same or come from the same place, for example.

In this kind of files, the colour profiles in use cannot be not included in the document. They must be referenced by tags pointing to an external and known location on the server, where they must stored. Not even the colour profile that defines the printing aim (the output intent) can be included.

As these closed systems do not receive materials from outside, the components are always the same, and are 100% under control. This allows transfering a huge amount of colour profiles, with its subsequent weight, making their management more efficient.

Standards that define it

PDF/X-4 is specified in ISO ISO 15930-7:2008 and ISO 15930-7:2010 standards.