Why file preparation matters
An office printer is forgiving. It prints on A4, in screen colours, with a margin, and rarely produces surprising results. Commercial print production is entirely different.
Offset printing uses sheets larger than the final product, which are then cut. The guillotine cuts stacks of paper with a tolerance of 0.5–1 mm. Colours are applied using four inks (CMYK), while a monitor - which emits its own light - displays colours differently from paper, which reflects them. Fonts installed on your computer do not exist on the printer's computer.
Each of these differences generates a class of errors that can be completely avoided by knowing how to prepare the file. Below, point by point.
Bleed - what it is and how much to add
Bleed is the area of the design that extends beyond the final print format. If the design has a background or image reaching to the edge of the page, it must extend beyond the cut line by a few millimetres.
Why? The guillotine cuts stacks of paper. With several dozen sheets at once, the bottom sheet may be shifted relative to the top by 0.5–1 mm. On a design without bleed, that shift produces a white stripe along the edge of the finished piece. With 3 mm bleed, the same shift is invisible because the background is exactly where it should be.
How much to add? This depends on the type of product. For simple items such as flyers, business cards, posters and typical marketing materials, the industry standard is 2 mm per side - the guillotine cuts them as single sheets or thin stacks and the error tolerance is small. For more complex products: catalogues, books, brochures with binding, board and packaging, 3 mm is used because thicker stacks and additional finishing operations generate greater shift tolerance. For perfect binding (glued spine), the bleed under the spine should be as much as 5 mm, because the paper shifts more during folding and gluing than with a simple cut. Always ask the printer before preparing the file rather than correcting a finished design.
What this looks like in practice: A 90×50 mm business card with 3 mm bleed means a file measuring 96×56 mm. The background and artwork run to the edge of that file (96×56), but text and logos must be at least 3–4 mm inside the cut line (90×50) so they are not clipped.
Bleed applies to every edge where a colour or image ends. It does not apply to edges with a white margin - there is no need for bleed there because there is nothing to extend.
Safety margin - the zone you do not cross
This is the other side of the same principle. Bleed protects against white strips. The safety margin protects text and important elements from being clipped by the guillotine.
Important design elements - text, logo, phone numbers, addresses - should all be at least 3–4 mm inside the cut line. This is the safe zone from which the guillotine cannot remove anything important even at maximum shift.
A design with text at the very edge of the printed area looks unprofessional even without production errors. With production errors it loses part of its content.
CMYK vs RGB - why your monitor lies
This is one of the most common sources of surprise after receiving a print. The client designed in RGB, it looked great on screen, but the print is "flat".
RGB is an additive model. The monitor emits light - it mixes red, green and blue light to create colours. Full intensity of all three channels produces white. RGB has a very wide gamut (colour range) and can display neon colours, vivid fluorescents, intense cyans and greens.
CMYK is a subtractive model. Ink on paper does not emit light - it absorbs it. By mixing cyan, magenta, yellow and black, we cover paper with layers that subtract successive components of white light. CMYK has a narrower gamut than RGB. Some colours that look great on screen simply cannot be reproduced with any combination of printing inks.
The most problematic colours when converting RGB to CMYK: a vivid orange (RGB 255/128/0) becomes a muddy brown after conversion. Intense neon green turns olive. Purple (RGB 128/0/255) loses intensity and shifts towards blue-pink.
Critical error: Designing in RGB and only converting to CMYK at export or leaving it to the printer. Automatic last-minute conversion gives worse results than manually controlling colours in CMYK from the start. Colours that looked intense in RGB can look flat and dirty after conversion. If the design is for print, work in CMYK from the very first pixel.
When the printer needs to know about brand colours
If the design contains brand colours that must be reproduced precisely (e.g. a Pantone corporate colour), always provide the printer with exact CMYK values or the Pantone number. Automatic conversion cannot guarantee hitting a specific shade. In offset printing, a Pantone ink can be used as a fifth colour, which provides a level of precision impossible with CMYK alone.
Total ink coverage - the deep black trap
CMYK has four channels, each from 0% to 100%. Total ink coverage (TIC) is the sum of all four channel values for any given point in the design. At CMYK 100/100/100/100 the TIC is 400% - and that is a problem.
Excessive ink coverage means the paper cannot absorb that much ink simultaneously. Effects: longer drying times, ink offsetting onto the reverse side (setoff), paper wrinkling and feeding problems.
Acceptable maxima depend on paper type. For coated papers (gloss or matt coated), the standard is 280–300%. For uncoated offset papers, it is safer to stay at 240–260%.
Rich black - when K=100% is not enough
Black made from the K channel alone (C0 M0 Y0 K100) can look slightly grey and flat on large surfaces. For large black backgrounds, "rich black" is used to give a deeper, more intense effect. A typical recipe: C30 M30 Y30 K100.
Important: Rich black has a TIC of 190%, which is safe. But text and fine lines should never be set in rich black. Text at C30/M30/Y30/K100 is printed with all four inks simultaneously - even minimal register misalignment produces blurry, illegible print. Text must always be: C0 M0 Y0 K100.
Practical rule: Rich black for large background areas (covers, banners). Pure K=100% for text, fine lines and small graphic elements.
Resolution - 300 DPI and why it makes sense
DPI (Dots Per Inch, often used interchangeably with PPI - Pixels Per Inch) defines the information density in an image. Web graphics typically have 72 PPI - sufficient for a screen. In print, 300 DPI is the standard.
Why 300? The human eye at a normal viewing distance (30–40 cm) can resolve detail up to approximately 300 dots per inch. A 300 DPI print looks sharp with no visible pixels. A 72 DPI print at 30 cm is visibly pixelated.
When can you go below 300? Large-format print (banners, large-format posters) is viewed from greater distances. For a banner viewed from 3 metres, 150 DPI is sufficient; for a billboard viewed from 10 metres, 30–72 DPI is enough. The larger the format and the greater the viewing distance, the lower the resolution requirements.
The most common error: Downloading an image from the internet (72 PPI) and placing it in the design. It looks fine on screen because the monitor has exactly the same resolution. In print it looks like it was built from Lego bricks. Enlarging an image in the design software does not increase its resolution - it only reduces the number of pixels per inch of print.
How to check resolution in Adobe InDesign: View → Display Performance → High Quality Display, then Preflight will flag images below the threshold. In Photoshop: Image → Image Size - check the Resolution value under Document Size in inches.
Fonts - why the printer may not have your typeface
Fonts are separate software, installed on a specific computer. An InDesign file opened on a computer that does not have the typeface installed will substitute it with a default - usually Arial or Times New Roman. The entire layout "breaks" - letter spacing changes, line breaks shift, text flow changes.
Two solutions:
1. Convert text to outlines (Create Outlines). Letters become vector objects and no longer require the font to be installed. They will print identically on any machine anywhere in the world. Disadvantage: text converted to outlines is no longer editable. Correcting a typo requires going back to the working file before conversion. Always keep a copy of the file with live text.
2. Embed fonts in the PDF. When exporting to PDF, tick "Embed All Fonts". The font is then bundled into the PDF and the printer does not need it installed. This is the more convenient option, as the PDF remains editable (in terms of text) until the last moment.
Minimum type size
In print there is a physical limit below which text becomes illegible or breaks up. The general rule: body text should be no smaller than 8 pt (for regular weight fonts) or 6 pt (for bold). Very thin fonts (thin, ultralight) may have problems even at 10 pt because their fine strokes may not be reproduced by the CTP plate and rubber blanket. For special requirements, always ask the printer for a press proof.
What is a press proof? A test print made before the main production run that allows you to check how the design will look after printing - colours, detail, text, finishing. There are two types. A digital proof is a print on a specialised proofer calibrated to match the colours of the specific press and paper, with an ISO compliance certificate. Faster and cheaper, it is sufficient in most cases. A press proof is a print made on the actual production machine, on the actual paper, with the actual inks - the most accurate test possible, but it requires full press makeready and is therefore costly.
A proof makes most sense for large, expensive runs where an error after production starts means losing all the material and the time for a reprint. It is also used for specialist work: packaging with precise brand colours, materials with metallic ink or special varnish, the first run of a new product, and printing on non-standard substrates. For standard jobs - flyers, brochures, posters at reasonable run lengths - a proof is generally not necessary if the file is correctly prepared.
File format: PDF/X-1a and PDF/X-4
PDF exists in many versions. A special family of standards called PDF/X was created for print production, guaranteeing that the file contains all the information needed for production - and nothing the printer does not need or cannot handle.
PDF/X-1a is the older, more restrictive standard. It does not permit transparencies - these must be "flattened" before saving. It does not allow colour spaces other than CMYK and Pantone. It is as safe as copying a file to a floppy disk: what you see in the preview is what you get in print. A good choice for simple jobs without complex special effects.
PDF/X-4 is the newer standard, supporting transparencies and layers. It is compatible with modern presses and digital workflows that handle transparency flattening themselves. For complex designs with special effects, shadows and overlapping objects, PDF/X-4 gives better results.
Practical rule: If in doubt - PDF/X-1a is safer. Flatten layers and transparencies before exporting, set 300 DPI for flattened objects. If the printer has a modern workflow (ask) - PDF/X-4 with preserved transparencies will give cleaner results.
Other export considerations
When exporting to PDF, check that bleed is included (Marks and Bleeds → Use Document Bleed Settings). Check that the colour profile is embedded or that colours are converted to the target CMYK profile. Check whether crop marks are included - though many printers prefer a file without trim marks and add them themselves at the imposition stage.
Transparencies and special effects
Shadows, blurs, gradients, objects with transparency - all of this is handled by modern DTP software without problems. But when saving to PDF/X-1a, all these effects must be "flattened" - converted to a static bitmap.
Problems arise when flattening is not done correctly. Effects may look different from the design. Text sitting on a transparent object can lose sharpness (it becomes bitmapped with its surroundings instead of remaining as a vector). Dark objects can pick up colour from the background during flattening, creating artefacts at edges.
Solution: check the flattened transparency preview in InDesign before exporting (View → Flattener Preview). If anything looks different from intended, adjust the flattening settings or reconstruct the effect differently.
Overprint - the trap that is invisible in the preview
Overprint is a property of an object that tells the press: "do not knock out what is beneath this object - print on top". Under normal conditions, every object knocks out what is below it. With overprint enabled, it is applied on top of and mixed with the underlying colour.
Black text should always be set to overprint (overprint K). This eliminates the problem of white halos around letters when there is minimal register misalignment. Most DTP software sets this automatically for black text.
The problem: if white text or a white object has overprint accidentally enabled, it is invisible in print - white ink does not exist (it is not printed), so the object disappears. In the screen preview it looks correct; in print it is simply not there.
Always check the overprint preview before final save: View → Overprint Preview in InDesign or Acrobat.
Print-ready file checklist
Before sending to the printer, check:
- File size matches the order, with bleed added on all relevant sides (2–3 mm depending on product type)
- All important elements (text, logo) at least 3–4 mm inside the cut line
- Colour mode CMYK - no RGB or Pantone objects (unless spot colours have been ordered)
- Total ink coverage does not exceed 280–300% for coated papers, 240–260% for uncoated
- Black text: C0 M0 Y0 K100 (not rich black)
- Large black backgrounds: rich black (e.g. C30 M30 Y30 K100)
- Resolution of images and bitmaps: min. 300 DPI at 1:1 scale
- Fonts converted to outlines or embedded in the PDF
- File saved as PDF/X-1a or PDF/X-4
- No accidental overprint on white elements (check with Overprint Preview)
- Transparencies flattened (for PDF/X-1a) or preserved (for PDF/X-4)
- File opened in Acrobat and visually checked - particularly first and last page
When it still pays to call the printer
Standards apply to standard jobs. But print is production, and every production has its nuances.
It is worth consulting the printer before preparing the file when: the design has very dark backgrounds with overprint, you are using lamination films or UV varnishes and want to confirm substrate compatibility, the design has very fine lines or small text and you are not sure they will reproduce, you are ordering something for the first time on a new paper or with a new finishing process.
A good printer will check the file before accepting it. A bad printer will print whatever it receives and send the invoice. You know which approach you prefer.
Summary in one sentence: Preparing a file for print is not bureaucracy. It is a set of rules that exist because paper, ink and the guillotine behave differently from a monitor and an office printer. Understanding those differences eliminates 90% of problems before they ever reach the press.