The PDF/X-1a standard

This is the first version of the PDF/X standard to be successful (previous versions were never really used). It is the level of PDF/X most compatible with outdated machinery or workflows that cannot handle transparencies and colour management, hence its success and why it is still in use. The general recommendation is that, whenever possible, its use should be avoided and the PDF/X-4 level should be used instead.

This level was published in 2001 and revised in 2003. Therefore there are two revisions:

• "PDF/X-1a:2001"
• "PDF/X-1a:2003".

For practical purposes there is not much difference between them.

Prohibitions and obligations

The prohibitions and obligations are what define a PDF/X level. These are the main ones in PDF/X-1a. 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 a PDF format level higher than 1.3 is forbidden (2001 revision). In the 2003 revision, level 1.4 is allowed, but in an extremely limited way (most of its features are not used). In both revisions, the use of level 1.2 is allowed (although not recommended).

  The use of features not included in the PDF format definitions of the level in use (e.g. third-party extensions) is prohibited.

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

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

  Attached files are not allowed (unless they are fonts or images that are a direct part of the content).

• Metadata: Every document must have the following metadata in its info dictionary:

  • Internal tags ("GTS_PDFXVersion" and "GTS_PDFXConformance") that identify the level and revision of the PDF/X standard for which the document was created.
  • Its creation date.
  • The modification date.
  • The document title.
  • A trapping key indicating whether the document has been trapped or not. The value "unknown", which can be used outside the PDF/X standards, is not allowed.

  In addition, every document must have a unique document ID.

• 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 prohibited.

• Optional content: Layers are not allowed.

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

  Embedding of OpenType fonts is forbidden.

• Transparencies: Transparencies (blending modes such as multiply, raster, etc.) are not allowed. All content must be flattened.

• Colours and Inks: The use of colours other than DeviceGray, DeviceCMYK, or spot colours (defined as Separation or DeviceN colour modes) is prohibited.

  This means that device independent colours (CalGray, CalRGB or with ICC colour profiles) and DeviceRGB, Lab or NChannel are not supported. 

  Smooth shading and patterns are allowed as long as they use allowed device or spot colour modes.

  If spot colours are used, the values of the alternate colour space of each one of them must always be the same in all the cases it is used.

  Besides, the alternative colour space of a spot colour can only be of type DeviceCMYK or DeviceGray.

• Compression: only a few algorithms are supported: JPEG, ZIP, CCITT (type 3 and 4), Run-Length (RLE), and ASCII85. LZW, JBIG2 and JPEG2000 are forbidden.

  Objects streams cannot be compressed.

• Images: Images with more than 8 bits depth per pixel are not allowed.

  The use of alternative images is not forbidden, but they must not be the default for printing.

• Pages and dimensions: Page geometry must be defined using the MediaBox, TrimBox or ArtBox. The BleedBox is optional and must be only used if there is a bleed printing.

  Page boxes must be correctly nested.

  The simultaneous presence of TrimBox and ArtBox is prohibited. There can be only one.

  Different page sizes are not allowed in a multi-page document (any box page must be the same  size throughout the whole document).

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

  PostScript code and transfer functions are not allowed.

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

  The use of scaling factors is forbidden.

• Printing aim: It is mandatory to report the final printing system, inks and paper for which the document has been created. This must be done through a structure called "output intent", explained in more detail on another page.

  In this level of PDF/X. the output intent is basically informative, as all colours contained in a document are either device (CMYK or greyscale) or spot (Separation or DeviceN). and therefore already converted to their final destination.

  It is forbidden for a document to have more than one output intent (which PDF format does allow).

  It is possible to specify the final printing system just with a reference to a set of standardised printing conditions, such as "FOGRA27" and a colour register (such as www.color.org).

  But if a colour profile is used (referenced by name or embedded in the document), that colour profile must be a CMYK one, printer's ("ptr") type and no higher than level 2 of the ICC specifications.

Standards that define it

PDF/X-1a is specified in the standards ISO 15930-1:2001 e ISO 15930-4:2003.