Preflight: How to avoid ten common problems of press-ready PDF files

April 2020

Overprinting, wrong bleed and RGB images are among usual mistakes when sending files to print. Czech graphic designer and publisher Filip Blažek has put together a preflight cheat sheet to save you time, hassle, and of course money.


By Filip Blažek

You’ve designed something stunning and you want the printer to turn it into a real product. Now it’s time to check all documents before sending them to the printer. This process is called “Preflight” and can save you a lot of trouble and, yes, even money. The following list describes ten of the most common issues printing houses deal with when receiving files from their customers. By following these simple rules, you can send flawless data to a printer and, instead of worrying about the results, enjoy a good cup of coffee or tea.

Both Adobe InDesign and Affinity Publisher provide decent preflight tools. The first step is to create a preflight profile — a list of problems you want to be warned about. The Preflight panel will then display all potential problems within the document. The default profile unfortunately dose not point out every mistake, but once you define your own profile it can become a very powerful tool.

You can also check your PDF in Adobe Acrobat, which offers an extensive set of Print Production Tools including Output Preview and Preflight. Nevertheless, don’t rely on software tools alone. Checking each page with your own eyes is essential!

Regular PDF files intended for online distribution do not match stringent professional printing requirements. So when sending files to professional digital or offset printing shops, make sure you are sending a print-ready or press-ready PDF. It is recommended to export PDFs according to one of the printing industry standards: select either PDF/X-1a or the newer PDF/X-4a. Consult the printers’ websites to find their own requirements.

Simple enough. Now the details before your files take flight.


1. Missing bleed
The missing bleed is perhaps the most common mistake printers encounter. The purpose of the bleed is to make sure your objects reach the very edge of the page without leaving any white stripes on the side. This rule applies to all elements: photos, illustrations, rules, and simple colour backgrounds. Always check whether the objects placed on the edge of the page do have bleed, so they extend 3 mm (0.125″) beyond each page edge (total of 6 mm or 0.25 inches).


In rare situations when there would be a risk of trimming an important part of the image, you can add the missing bleed in Photoshop using the stamp tool, for example.


2. Objects placed too close to the page edge
Most printed matter is cut down to the final dimension after printing. It is a mechanical process and as such it is subject to impreciseness. If you place the text frame too close to the edge of the page, it can be partially trimmed. The same applies to thin lines around a page: the final result can lead to slightly different line widths on each side. For stationary, laser cutting might be a better option since it is more precise.


Text aligned to a page edge or very thin frames can lead to disappointment. The intended design on the left may get trimmed to look like the unintended design on the right: The left and bottom edges are cut too much.


When printing multi-page documents, the print shop first imposes pages on a large sheet of paper called the signature. Those signatures are then printed, folded, and trimmed. Depending on the thickness of the paper, the real width of each page can differ between the outer and inner spread by two millimetres (0.08″). Therefore, avoid placing objects such as page numbers too close to the edge of the paper because the changing distance from the edge would be too visible for the reader. Generally speaking, the safe area, in which you can place important contents, is usually at least 5 mm (0.2″) from the edge of the page.


Placing page numbers too close to the edge of the page can cause them to be trimmed inappropriately.


3. Text colour defined in RGB
Professional printers generally use four process colours — cyan, magenta, yellow, and black (CMYK) — and spot colours. All objects defined in RGB colours have to be converted into CMYK before printing which may lead to an unexpected colour shift.

If you keep black text in RGB, it won’t be reproduced using 100% black colour in a single pass, but by all four CMYK colours one at a time. Four colors, four passes. The likelihood is that each pass will not print precisely on top of the previous one, so the printed CMYK text will look slightly blurred and brownish. Therefore, standard black text should be printed only by 100% process black so it remains sharp. Always use the print or press-ready PDF export option to make sure colour won’t be converted to RGB.
Please note that some printing houses support PDF/X-4 standard and RGB workflow. In those cases you can keep images or illustrations in RGB.




4. Wrong overprint settings
Each object placed into your document will either overprint all elements below itself or knock them out. Knockout is the default setting for every colour or image with the exception of 100% black.

In InDesign, the overprint preview is off by default so you’ll have to turn it on when checking your document. Unfortunately, Publisher does not offer overprint preview at the moment, but there’s an easy workaround. Export the PDF first and then check it in Acrobat with Overprint Preview turned on.


 The screen preview may not be WYSIWYG, which can lead to overprint or knockout. Always check your files with Overprint Preview turned on.


 Knockout: The cyan letter does not overprint on the yellow background, keeping its blue colour.


 Overprint: Overprinting a cyan letter on a yellow background results in a colour change to green.


5. Forgotten spot colours
If you keep spot colour(s) in a document intended for printing using CMYK process colours, they will be converted into their CMYK equivalent which may lead to a sad surprise. For instance, the metallic gold Pantone colour may look like an ugly shade of brown on the paper.

To avoid unintended results, make sure there are no unwanted spot colours in your document before generating the press-ready PDF. Pro tip: spot colours can be part of imported artwork, so you may need to adjust linked files too.
Please note that some printing houses support PDF/X-4 standard and RGB workflow. In those cases you can keep images or illustrations in RGB.


6. Odd or wrong number of pages
Some clients do not realize the total number of pages of a brochure must be divisible by four. In the case of magazines or books, the printing press may require page numbers divisible by 8 or even 16. If you get a request to design a 14-page document, just realize you’ll have to send a 16-page PDF to the printer anyway.

Always keep the technological requirements in mind, especially during the early design stage. Removing two pages from a 386-page document in the very last moment may not seem like big deal, but it can be troublesome.


7. Wrong document dimensions
Always check whether the document dimensions match the finished page size. Imposition software used by printers rely on the “Trim Box”, a definition of the final page size embedded in the PDF by desktop publishing apps.

If you place a business card in the middle of a large A4 or letter-sized document (even if you include crop marks and bleed), the imposition software cannot process the file easily. Send only individual pages to the printer; do not send spreads or try to impose pages in your app. Printing houses will deal with imposition on their own.


Left: The red line represents correct Trim Box.
Right: The Trim Box (red line) does not match the expected final document dimension.


8. Fonts too small, rules too thin
Every technology has its limits, so avoid extremely thin lines and very small font sizes. While offset printers can handle black 0.1 pt rules perfectly, digital printers may fail to print them. Very small texts or thin rules printed in colour or shades of gray can be partially or completely illegible. If you don’t have enough experience with printing, submit your design for approval to the printer before placing your order.


A comparison of a 9 pt letter enclosed in a 0.25 pt frame printed by 100% black, compared to 40% black using 150 lpi halftone. If you magnify the same object, you can see the halftone is too low for legible and good reproduction of the object.


Please note that due to the absorption of paper and printing technology used, the ink may slightly spread when it contacts the paper. This effect called “dot gain” can cause problems with thin, white text on a black background, especially when printing on uncoated paper. In this scenario very thin type can simply disappear.


9. Wrong colour profile
If you design an ad with a photo in it and have the same PDF printed on coated, uncoated, and newspaper paper, each photo will be reproduced differently. For example, coated paper has a layer on top that makes ink remain on the surface instead of being absorbed. Uncoated or newsprint doesn't have that layer, so ink gets absorbed quickly. This absorption rate changes how colors appear, the crispness of images, and other important details. To reproduce the colours as consistently as possible, you have to convert RGB images into CMYK using three different colour profiles (ICC profiles).

You could convert the photo in a bitmap editor, but as a result you will have three different image files (intended for coated, uncoated, and newsprint paper). It is more convenient to select the right colour profile during the PDF export process from InDesign or Publisher. Ask your printer for the correct profile for your job before sending them the files.

Unfortunately, the preferred colour profile depends on the region in which you live and how advanced the printing house is. At the top level, Adobe ignores the requests to include the newest standard European colour profiles in their Creative Cloud apps, so when dealing with European printers and designers, you will have to install them manually from the European Color Initiative website.


Paper with higher ink absorbency requires less ink, so CMYK values depend on the type of paper used. ICC profiles allow correct conversion of colours with respect to the total amount of ink used on coated, uncoated, newsprint, and other materials.


10. Text box too-small
Check your document for the little red “plus” or arrow icon at the bottom right of a text frame. This indicator shows when a text box is too small for the text it contains. It could simply be an empty line at the end of your text or it could be the byline for the famous writer you are quoting. Fortunately, both InDesign and Publisher will warn you before exporting such PDFs (unless this warning is disabled).


A symbol of overset text in InDesign (left) and Publisher (right).


As you can see, preparing for print isn’t magic, just follow a few simple rules. Now that you’re ready, let’s get your project to the printer!

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