What's in a file?

Confusion abounds when people talk about file formats:
Which one should I use? But my printer told me to always use this format? I heard that the other format isn't good. Illustrator's native file format is PDF. InDesign can read native Illustrator files. EPS is dead to me. Always save your file as EPS. PDF solves all problems. PDF results aren't high quality...

But the real question you should be asking is:
What is actually *IN* a file anyway?

If you understand what a file is, and what's in it, you can answer all of the questions above, and then some. Several years back, I posted something like this to the PrintPlanet CTP list. But it's wonderful information to have in any case. You just might want to print this out, shrink it down, and tattoo it to the back of your hand, for easy reference :)

Illustrator Files
When you save files from Illustrator (either using the Save or the Save As command from the File menu), the three mainstream choices you have are Adobe Illustrator Document (.ai), Illustrator EPS (.eps), and Adobe PDF (.pdf).

- Adobe Illustrator Document (.ai)
This is a native Illustrator format. Only Illustrator is able to read native data. While this data is based on the PDF Langauge specification, it isn't a file that any other application can read (including Acrobat, InDesign, or Photoshop, etc). This format retains ALL editability in your file, and is the one you should always internally for your own use (for use, archiving, storage, etc.). You may have heard that native .ai files are PDF or that InDesign can read native .ai files. That isn't true. What *IS* true is that when you save a native Illustrator file, Illustrator also include a PDF 1.4 composite of that file inside the file as well. So a native .ai files isn't a PDF file, a native .ai file CONTAINS a PDF file.* The PDF 1.4 file supports transparency, so both the native .ai portion and the PDF 1.4 portion of the file are both in an unflattened state. It should also be noted that the native portion of the file that is saved is obviously saved for that version of Illustrator (each version of Illustrator has its own native version). So when you're saving your file out of Adobe Illustrator CS2 as an Adobe Illustrator Document (.ai), you get a single file that contains:
- Native Illustrator CS2 content (unflattened) - used when file is reopened in Illustrator
- PDF 1.4 content (unflattened) - used when file is opened or placed anywhere else*

*In the Illustrator Options dialog that appears when you save a native .ai file, a checkbox called Create PDF Compatible File, marked on by default, determines whether the PDF portion of the file is included when the file is saved. With the option turned on, your file size grows, but the file can be read by apps like InDesign and Photoshop. Turning it off will chop the file size in half (and speed up save time), but the file will only be able to be reopened in Illustrator.

- Illustrator EPS (.eps)
Standing for Encapsulated PostScript, EPS is a format that is supported by a majority of graphics applications. PostScript does not support transparent constructs, so an EPS is a flattened file format. Illustrator can interpret (or parse) EPS content into its own native format when opening files, and it can write or convert its own native format to EPS as well. In that translation, flattening occurs, and general editiablity is lost as well (effects are expanded, text is broken apart, etc.). Therefore, Illustrator will also include a native .ai version in the file, so that should you ever reopen the file in Illustrator CS2 again, all of your artwork will be fully editable.* So when you're saving your file out of Adobe Illustrator CS2 as an Illustrator EPS (.eps), you get a single file that contains:
- EPS content (flattened) - used when file is opened or placed anywhere else
- Native Illustrator CS2 content (unflattened) - used when file is reopened in Illustrator CS2*

*Remember that Illustrator saves its native content for the version that you specify. So if you save your file out as an EPS file compatible with Illustrator 8, then the native Illustrator data that is saved along with the file is Illustrator 8 data -- a format that didn't support transparency. Also, saving back to previous CS versions mean you're going back to the pre-new text engine versions, and text won't be editable, even if the file is reopened in Illustrator CS2.

- Adobe PDF (.pdf)
While Illustrator's native file format is based on the PDF language specification, there are many constructs that Illustrator uses that aren't supported in PDF directly (reflowable text, styles, effects like 3D, blends, etc.). So when you save your file as a PDF, Illustrator writes its data out so that it can be read in a format that any PDF reader (or app that can place PDF) understands. To retain full editability upon reopening the file in Illustrator, a native CS2 version of the file is also saved inside the file.* So when you're saving your file out of Adobe Illustrator CS2 as an Adobe PDF (.pdf), you get a single file that contains:
- PDF content (flattened if you choose PDF 1.3, unflattened otherwise) - used when file is opened or placed anywhere else
- Native Illustrator CS2 content (unflattened) - used when file is reopened in Illustrator*

*In the Save Adobe PDF dialog that appears when you save a PDF file, a checkbox called Preserve Illustrator Editing Capabilities, marked on by default, determines whether the native CS2 portion of the file is included when the file is saved. With the option turned on, your file size grows, but the file can be reopened in Illustrator. Turning it off will chop the file size in half (and speed up saving time), but the file won't be very editable if reopened in Illustrator. As an aside, since we usually send PDF files to clients anyway, and not only do we want smaller file sizes so they email faster, but we have no interest in giving the client the ability to open the file themselves for editing in Illustrator, turning this option off makes a lot of sense. However, when sending files to printers, leaving the option on means that they can reopen the file in Illustrator to make tweaks or adjustments if necessary.

PDF Versions
While you can save PDF files from Illustrator, there are different versions of PDF. In reality, there are two variables here. There are versions of Acrobat, and there are versions of the PDF Language specification (referred to by us techie people as PDFL). Just in case you get confused, an easy way to remember which PDFL goes with which version of Acrobat, is to just add up the numbers (PDFL 1+4 = Acrobat 5, etc.)

- Acrobat 4 (PDFL 1.3) - predated the days of transparency. So in terms of flattening, think of PDF 1.3 as though it were EPS (with a few added benefits). PDF 1.3 files are always flattened. PDF 1.3 does support CMYK and Spot Colors. This version also introduced smooth shading technology into PDF and digital signatures.

- Acrobat 5 (PDFL 1.4) - first version of PDF to support transparency. When you save a native .ai file from Illustrator, the PDF that Illustrator embeds in the file so that other apps can read the file, is PDF 1.4. Besides support for transparency, this version also introduced XML-tagging and metadata support. A PDF 1.4 is not a flattened format. The only way to get flattened content into a PDF 1.4 file is to manually flatten the content within Illustrator, on the artboard before you save it.

- Acrobat 6 (PDFL 1.5) - probably the most popular version of Acrobat reader and app installations today. Obviously, an unflattened format, PDF 15. introduced the concept of PDF layers, and allows JPEG2000 compression.

- Acrobat 7 (PDFL 1.6) - Same as 1.5 but has added object-level metadata support, and AES encryption.

- Acrobat 8 (PDFL 1.7) - Hot off the press, Acrobat 8 only started shipping a short time ago. Illustrator CS2 can't save in this format, but future versions of Illustrator should be able to.

PDF Standards
In an effort to achieve some kind of level playing field for PDF files, several standards have been established. These are all regular PDF files, but a PDF can be validated to meet the requirements defined by these standards.

- PDF/X-1a - has become the standard for PDF file within the printing and publishing community. Among other things, PDF/X-1a requires that all fonts are embedded, transparency is flattened, colorspace is CMYK and/or spot, and that the file is PDF 1.3 compatible.

- PDF/X-2 - created specifically for OPI workflows, where hi-res data is swapped in for FPO data at print time. This is problematic in transparency workflows, as hi-res data is needed at flattening time. Highly specialized, this format is used mainly in packaging workflows.

- PDF/X-3 - seen as the "next" step for printing workflows, PDF/X-3's main difference from PDF/X-1a is that RGB data is allowed, on the condition that an intent profile is present. This allows printers to attain greater control over their own color conversions and color integrity, as the conversion from RGB to CMYK happens on their watch -- not the designer's.

- PDF/X-4 (Draft) - still in draft form, the main attribute of PDF/X-4 is that it will allow transparency. This would obviously enable the printer to handle the flattener settings.

- PDF/A - I doubt anyone reading this will ever have use for this, but PDF/A is a standard that has been introduced to assist in the archiving of data. If you think about it, there are some files that we may have worked on 10 years ago, which aren't supported in any apps today. While the file may be sitting on a CD somewhere (or most likely, a Syquest cartridge), you don't have any app that can open it. PDF/A is a standard that was established to ensure that today's files will be accessible in the future. The largest group of people who utilize this standards are those in the government, and in medical and insurance fields, who need to archive huge amounts of data electronically.

So that brings us to the end of our discussion. Hopefully we've all learned a little something today. Now go save some files!