Guide
Your finished piece is A4, but the artwork is bigger. That extra area is bleed.
A4 is 210×297 mm. Add 3 mm bleed on every side and the file becomes 216×303 mm. Finished size and artwork size are not always the same.
An A4 job can legitimately be 216×303 mm
When artwork runs to the edge, printers often ask for image or background beyond the trim line so a tiny cutting shift does not leave a white edge.
With 3 mm bleed on all four sides, 210×297 mm becomes 216×303 mm. Three millimetres is only an example — always use the printer's actual specification.
Background goes out; critical text comes in
Bleed extends backgrounds beyond the trim. Logos, text, page numbers, and anything you cannot afford to lose should move the other direction, inside the safe area.
Treating background art and critical content the same way near the edge is an easy source of print surprises.
Decide the physical dimensions before converting to pixels
DPI/PPI connects physical size to pixel dimensions. Once the bleed-inclusive millimetres are known, convert that size at the required resolution.
Changing only an image's DPI metadata does not create new pixels. What matters is whether the available pixel count is sufficient for the intended physical size.
Check the PDF page size, not just the zoomed preview
A file can look sharp at 200% and still have the wrong physical page dimensions. Inspect the PDF page size together with trim, bleed, and crop marks.
For booklets, page multiples, cover handling, and imposition are separate questions. Keeping those checks separate makes preflight much easier.