A trim size is not a page size.

The page is the trim plus the bleed on every side, so an A4 job with 3 mm of bleed is a 216 mm × 303 mm PDF. And a PDF is measured in points, which are exactly 1/72 inch — 25.4 mm to the inch makes a point 0.352778 mm — so nothing metric is a round number of them: A4's trim is 595.276 × 841.89 pt and its page is 612.283 × 858.898.

The engine is free and needs no account. And “fit to page” is the enemy: anything offering to fit that 216 mm page onto 210 mm of paper scales it to 97.22% — the artwork shrinks, every element moves inward, the crop marks land 2.92 mm inside the trim rather than on it, and the finished piece comes out 204.17 mm × 288.75 mm. The bleed was bought so the cut has somewhere to be wrong, and scaling spends it — after the file leaves you. Creep eats it too: 64 pages stitched is 16 folded sheets, and on 0.1 mm stock the innermost spread creeps 1.5 mm, which is half a 3 mm bleed before the guillotine has had a turn. And how you butt the pages decides the N-up, not how big they are: two pages sharing a trim edge share one cut and one bleed, so the pitch is 148 mm against 160 mm — on SRA3 that is 4-up against 2-up, half the sheet and twice the paper. Every lane ships a worked example that costs nothing. Writing the report needs credits.