Preflighting is the process of checking if the digital data required to print a job are all present and valid. Nowadays PDF files are sent to printing companies. The PDF file format is a solid standard to exchange pages, ranging from single ads to complete publications. Using PDF is, however, no guarantee that the receiver of a file can actually output it as intended. To make sure that a file matches the requirements to reproduce its correctly, it needs to be checked or ‘preflighted’. This page provides an overview of
- the reasons to preflight files
- the tools that are available to check page content
- who should do the check and when it should be done
- which settings are best used
In case you wonder: the term preflight was derived from the long list of checks that pilots have to complete before taking off with an airplane.
Why preflight PDF files?
Checking files is essentially done to avoid problems in processing or printing content. If you look at the list of the 10 most common problems with PDF files, half of them can easily be avoided by preflighting all files. Enfocus published a nice PDF about the 10 reasons to preflight files.
How to preflight PDF files?
There are a number of preflight solutions on the market.
- From version 6 onwards Adobe Acrobat Professional includes a preflight engine. With each new release, this option has been improved and it is actually pretty powerful in Acrobat Pro CC.
- There are several preflight plug-ins for Acrobat. The best-known ones are Enfocus PitStop and callas pdfToolbox.
- There are also stand-alone preflight applications on the market. Some like Enfocus PitStop Server focus specifically on checking PDF files. Others like Markzware FlightCheck can handle a wider range of file formats.
- Many prepress workflow systems, such as Agfa Apogee Prepress or Kodak Prinergy, include a preflight engine. It is either a module which was licensed elsewhere or the vendor’s own development.
Which engine you choose depends on the level of automation that is needed, the different types of files that the engine needs to be able to handle, the platform on which the application should run (Mac, PC,…) and the available budget. It is a good idea to use a solution that has been certified by GWG and appears in their application list. This way you are certain that the preflight system can correctly handle general market requirements.
Who should preflight when?
The earlier in a process that problems are detected, the easier and cheaper it is to fix them. This implies that designers should preflight their creations before sending them to their agency or printer.
Printers or service bureaus should always preflight incoming data, simply to make sure that the quality of the product that they will deliver meets the customer’s demands.
It is worthwhile to know that there are two technologies available to ensure that files only need to be preflighted once. These are Enfocus ‘Certified PDF’ and the Ghent Workgroup ‘Universal Proof of Preflight’. Essentially both systems allow a preflight application to embed metadata in a checked PDF to document how the file has been checked. The receiver of such a file only needs to verify if this ‘seal’ is present.
What needs to be checked during preflight?
The things that need to be checked in a file depend on its intended usage. This means that it is impossible to provide one single set of rules that apply to every possible type of job or printing. Below is a general description of what should be checked and why it is checked, including the reason why each check is done. The overview is inspired by the GWG 2015 specifications. This is, as far as I know, the only international standard that describes the requirements of print-ready PDF files. I’ve simplified their recommendations here and there to keep the overview from getting too long.
General file settings
- The PDF file must be a certain version. This is done to avoid that someone uses a brand-new application to create a PDF that no-one else’s system can read yet. The GWG-specs recommend that the PDF is PDF/X-4 compatible. This means that the file must be a PDF 1.6 or 1.7 file. These PDF versions have been around for more than 10 years so compatibility should not be a problem.
- Earlier GWG standards did not allow the use of transparency, layers (optional content, often used for multi-language jobs or other types of versioning) or color-managed color spaces. All of these are now allowed.
- There are a number of compression algorithms for PDF files. Some of the newer algorithms may pose problems in older workflows so it is best to check if this is the case. The PDF/X-1a file format that GWG recommends to use already includes a number of restrictions on the types of data compression that can be used.
- PDF 1.6 and later have a page scaling parameter that should not be used. To keep things straightforward, it is best to assume that a file is created 1:1 unless there are clear guidelines that are known by all parties involved.
- A PDF file can contain custom halftone definitions. Since such a definition can lead to output at the wrong screen ruling or with the wrong dot shape, it is better to check that the file has no custom halftone data.
- PDF files can contain annotations. Preflight needs to make sure that there are no annotations in the TrimBox. GWG also recommends to only allow annotations of the following types: Text, Link, FreeText, Line, Square, Circle, Highlight, Underline, Squiggly, Strike-out, Stamp, Ink, Popup, FileAttachment and Widget.
Page specific settings
- Page dimensions are described internally in PDF files using the so-called page boxes. Since it is not always possible to make sure that the PDF TrimBox actually equals the finished trim size of a publication, GWG sets no rules for the correct use of these boxes. They do recommend to check that no CropBox is present and that there are no page elements beyond the MediaBox. The first requirement ensures that users see the full page in Acrobat while the latter makes sure there is no useless ‘crap’ present in the PDF.
- The page size and orientation (as defined by the TrimBox) for all pages of a PDF file should be equal.
- Sloppy designers sometimes leave empty pages in their design. When each section of a book is a separate PDF file and one of these accidentally contains an empty page at the end, this can completely screw up the job. To avoid this, check that there are no empty pages in the file.
- When PDFs are used for advertising, it makes sense to only allow 1 single ad page per PDF. This ensures that agencies do not group multiple ads in a single file, making it too easy to accidentally place an incorrect ad.
- Files should not contain black text that is smaller than 12 points and set to knock-out. This is necessary because it is very difficult to print such text. Even the smallest alignment problem on the press can cause such text to become illegible.
- Check if the PDF contains white text set to overprint. Such text may be visible when the text is watched on-screen but it disappears in print.
- Very small text quickly becomes either illegible or difficult to print when it is colored. GWG recommends to flag any files with text smaller or equal to 5 points (8 points for newsprint). When it is colored with 2 or more colorants, text should not be smaller than 9 points (10 points for newsprint).
- All fonts should be included in the PDF file. This is done to avoid that an incorrect font is used, which could lead to text becoming illegible or part of the text disappearing. The inclusion of OpenType fonts is allowed.
- When a certain font is not available during the PDF creation, it typically gets replaced by Courier. To avoid printing a PDF with missing fonts, check that no Courier font is present in the file. Designers who want to use Courier on purpose can use variants like ‘Courier New’, which are not flagged during preflight.
- Images need to have a certain minimum resolution to be printed at a good quality. The required resolution depends on the type of printing. That is why the preflight process should check if all image resolutions exceed a minimum threshold value. GWG recommends rejecting files containing color and grayscale images with a resolution below 100 ppi for newsprint and 150 ppi for commercial offset work. For 1-bit images anything below 550 ppi is considered unacceptable. Many preflight applications can already give a warning when the image resolution gets dangerously close to these minimum values.
- Images that have a resolution that is too high lead to bloated files that take longer to transmit or process. For newsprint GWG sets the maximum resolution to 300 ppi for color or grayscale images and 1905 ppi for 1-bit images. For commercial print using offset presses, the maximum resolution is set to 450 ppi for color or greyscale images and 3600 ppi for 1-bit images. Defining a maximum resolution is somewhat controversial: if this problem gets resolved by downsampling images, this can cause issues with special types of images such as security elements. Some users also argue that large file sizes are no longer the problem that they once were. From their point of view, the risks of downsampling exceed the advantages of limiting the file size.
- By checking if white line-art is set to overprint, you avoid that such images disappear on the final output.
- It is possible to put images that use 16 bits per channel in a PDF file. This leads to larger files but that is no longer seen as an issue so 16-bit images are now allowed.
- If a design contains very thin lines, these may disappear on the printed result. Therefore it is best to check the minimum line weight, which depends on the intended printing process. For newsprint and commercial offset printing, a minimum line thickness of 0.125 points is required. For screen printing, 0.15 points is required.
- Check if grayscale objects aren’t set to overprint. Overprinting grayscale objects can lead to excessive ink build-up, not to mention that the graphic sometimes becomes soo dark that it cannot be distinguished from the background anymore.
- Total ink coverage is another important aspect of preflighting. As a general rule, newspaper jobs shouldn’t have any object in them for which the ink coverage exceeds 245%. For commercial printing on offset web presses, the ink coverage of any element should not exceed 305% while for sheetfed presses or screen printing that percentage is 320%. If CMYK text contains over 85% black ink, the total ink coverage of the text should not exceed 220% (newspaper) or 280% (web or sheetfed offset and screen printing).
- Even worse than having to cope with heavy ink coverage is dealing with text or other objects that are 100% of each printing color. Having to print 100% cyan on top of 100% magenta, yellow and black leads to smudging and the need to frequently stop and clean the press. Hence the recommendation that any object inside the TrimBox should not use the separation colorspace ‘All’.
- When a job is meant to be printed with spot colors, it is obvious that spot colors are allowed in a PDF file. Next to the presence of spot colors, it is best to put restrictions on their names (no confusing mix of suffixes, such as a file containing ‘Pantone 638 C’, ‘Pantone 638 CVC’ and ‘Pantone 638 CVU’) and color definitions (A PDF might contain a spot color which is defined as being 43C & 68M on one page and 40C, 63C, 2Y, 4K on another).
- A PDF/X file contains an output intent, which is a description of the intended color space when the file is printed. GWG recommends that this output intent is an embedded ICC profile. The ICC profile v4 format is supported and GWG no longer insists on the use of specific recommended profiles.