Bleed and crop marks

A knife never cuts exactly on the line. Whatever should reach the edge of the page has to run past it, so a small shift leaves no white strip. The @page descriptors bleed and marks add that room around the trimmed page:

@page {
    size: a5;
    margin: 2cm;
    bleed: 3mm;
    marks: crop;
    background-color: #dceefb;
}

Ask the print shop how much bleed it wants, 3mm is common.

The page grows, your layout stays

The PDF page grows by the bleed on all sides, and by 1cm more for the crop marks. The page you describe stays as it is and becomes the TrimBox, the bleed becomes the BleedBox. size is always the trimmed size, and margin counts from the trimmed edge.

Without an explicit bleed, crop marks come with 6pt, as CSS defines for bleed: auto. The marks print on every plate, including spot colors.

Both descriptors hold for the whole document, the @page rule of the first page decides.

What runs into the bleed

The @page background-color and background-image run into the bleed by themselves. A background image is scaled to the size of the sheet plus the bleed, so an image with its own bleed fits.

Elements stay on the sheet unless you send them past it. Absolute offsets count from the corner of the trimmed page, so a negative value reaches into the bleed:

.band { position: absolute; top: -3mm; left: -3mm; width: 154mm; height: 20mm; background-color: #c0392b; }

The width here is the A5 page, 148mm, plus the bleed on both sides. Stop at the edge of the bleed: anything further runs over the crop marks.

Check the boxes

pdfinfo shows the page boxes in points. The TrimBox is the size you asked for, the BleedBox lies 3mm (8.5pt) around it:

pdfinfo -box flyer.pdf

MediaBox:      0.00     0.00   371.34   493.23
BleedBox:     28.35    28.35   342.99   464.88
TrimBox:      36.85    36.85   334.49   456.38

For a file to hand over, set a PDF/X level as well.