We see it regularly: a client sends files to the printer and gets an email back saying they need to be redone before the job can proceed. Sometimes files come back from print looking different to screen. Almost every time, it comes down to one of the same four things.
Here's what a print-ready PDF actually requires, and why each element matters in practice.
1. Bleed
When a design element - a photograph, a background colour, a graphic - extends to the edge of the page, it needs to continue 3mm beyond the trim line. This is called bleed. Without it, even tiny variations in the cutting machine leave a thin white sliver at the edge of the page. On a publication with full-bleed photography, that sliver appears on every affected page of every copy printed.
The fix is simple but it needs to be set up at the start of a project. In InDesign, the document setup includes a bleed field. Set it to 3mm and extend any edge-to-edge elements into it from the beginning - not as an afterthought at PDF export.
2. Crop marks
Crop marks (sometimes called trim marks) are the small hairline crosses that sit outside the bleed area, showing the printer's cutting machine exactly where to trim. They're included automatically when you export a PDF with the correct settings checked. Without them, a printer has to guess the trim position, which introduces risk.
When supplying files, always export PDF/X-1a or PDF/X-4 with "Marks and Bleeds" settings enabled, crop marks switched on, and bleed values matching your document setup. Most printers will specify their preferred PDF export settings - follow them exactly.
Common mistake: Exporting a PDF in RGB "because it looks better on screen" and expecting the printer to sort the colour. They will convert it - but unpredictably. Always supply CMYK files for offset litho print.
3. Colour profiles
Screens display colour in RGB - three channels of red, green and blue light. Commercial printing uses CMYK - four inks, cyan, magenta, yellow and black. If you export a PDF in RGB, the printer will convert it to CMYK, and that conversion is not guaranteed to preserve your intended colours. Dark teals can shift green; rich navy can go flat and grey.
For European offset print, the standard is ISO Coated v2 (FOGRA39). Your InDesign document should be set to CMYK with this ICC profile assigned, and all placed images should be converted to CMYK before layout. Photographs supplied in sRGB are fine for digital use but need converting for print.
4. Image resolution
Photographs and graphics need to be at least 300dpi (dots per inch) at the size they appear on the printed page. An image that looks crisp on screen at 72dpi will print noticeably soft. This isn't something that can be fixed at the PDF export stage - the source file needs to be high enough resolution from the start.
When sourcing images for a print publication, always request files at the highest available resolution. For stock images, download the largest size available. For commissioned photography, request RAW or high-res TIFF files, not web-quality JPEGs.
A word on preflighting
Before exporting any PDF for print, run a preflight check. InDesign's built-in Preflight panel (Window → Output → Preflight) flags missing images, RGB images, missing fonts, and overset text. A clean preflight is not a guarantee of a perfect file - but it catches the most common problems before they reach the printer. We run a preflight on every file before delivery as a matter of course.