Frequently Asked Questions
Martin Bailey is senior technical consultant at Harlequin, a technology provider. Recently, Bailey sat down to outline the ins and outs of file formats and industry associations. The following is an excerpt from Bailey's discussion:
Q. Why do we need another file format? Isn't PDF enough?
Bailey: PDF/X is not an alternative to PDF; it's a focused subset of PDF designed specifically for reliable prepress data interchange.
Q. What can I do in PDF/X that I can't do in PDF?
Bailey: Nothing. The important point is that you can do a lot of things in PDF that are not appropriate for graphic arts use, and that can cause problems when outputting for high quality reproduction. PDF/X can be thought of as a shorthand way of specifying most of what you need to tell somebody in order for them to create a file that's likely to print correctly when they send it to you, even if they don't understand the details of what it's doing for them.
At one end of the scale are application files like QuarkXPress documents. You can change those in whatever way you like if you have the application. Unfortunately, the receiver of a file can also change them accidentally rather too easily, and the results you get when printing are dependent on many factors in the environment in which that copy of XPress is running, such as fonts and printer drivers.
In between, in order of decreasing flexibility and increasing reliability, other options at other positions on the scale include PostScript, EPS, PDF, PDF/X and TIFF/IT. When I use a name like "PostScript" in that list, I mean the format in an otherwise unspecified way. It's always possible to push such a format towards reliable end of the scale by using appropriate software to create it. In Northern Europe, many people use ProScript, which limits the options used in EPS files. A ProScript EPS file might be placed on the scale somewhere between PDF and PDF/X. In the same way, appropriate use of preflight tools on PDF can give you "reliable PDF" much closer to where PDF/X is on that scale. The point of PDF/X is that it gives you a convenient and well-specified label to use when asking for such a reliable PDF file.
Q. What's this about PDF/X-1 and PDF/X-2?
Bailey: The original intent of the standard was that it be split into two. PDF/X-1 is a file format for what's known as "blind exchange," where all technical information and content is held within one single file and nothing needs to be supplied alongside it, while PDF/X-2 will be a format for more open exchanges.
PDF/X-2 will address exchanges where there is more discussion between the supplier and the receiver of the file. The receiver may have certain fonts available which would therefore not need embedding, or maybe the receiver already holds high-resolution images to replace proxy images (low-resolution previews) in the supplied line.
PDF/X-1 is limited to colors defined in CMYK (and posts) only. PDF/X-2 will also allow device-independent color spaces, like Lab, to be used.
Q. Isn't PDF/X raster-only? It's just a wrapper for TIFF/IT, isn't it?
Bailey: Although it's possible to use PDF/X-1 as a wrapper for TIFF/IT files, that is not the intent of the design. A PDF/X file can—and usually will—include vector objects (such as rules, fills and text) using normal PDF constructs. It can also include image data, whether scanned or computer-generated. In this sense, PDF/X is very similar to both PostScript and PDF. Unlike PostScript, PDF/X-1 can make references to TIFF, EPS, DCS and TIFF/IT-P1 files and actually have external files embedded within it (although PDF/X-1a, PDF/X-2 and PDF/X-3 cannot).
Q. Where can I get more information?
Bailey: CGATS information is available at the Web site for NPES, The Association for Suppliers of Printing, Publishing and Converting Technologies: www.npes.org/standards/cgats.htm.
The DDAP Web site maintains a list of software either already available or being developed to support the PDF/X standards.