Get the size right.

The PDF specifies a physical size. Your printer settings decide whether that size survives onto paper.

1. Choose the same paper size and orientation in the tool and the print dialog.
2. Print only PDF page 1, the check sheet. Choose Actual size / 100%. Turn off Fit, Shrink oversized pages, Scale to fit and borderless enlargement. Do not print multiple PDF pages on one sheet.
3. Measure both lines between the end ticks with a physical ruler. Both must be 100 mm. A screen measurement is not a print test.
4. If either line is wrong, fix the print dialog or try another PDF viewer. Print the check page again before the poster sheets.
5. Print pages 2 onward with exactly the same settings. Keep the page numbers for assembly order.

Why “Fit” changes things

Fit rescales a PDF to the printer's printable area. A 100 mm reference might become 96 mm; the entire poster then shrinks. Setting a bigger margin in this tool can help a printer that clips edges, but it does not replace the 100% setting.

What size is the poster?

The width and height fields describe the image, excluding margins. A further 3 mm gutter inside each print margin keeps crop marks and page numbers away from the unprintable edge and off the image. The last row or column may contain extra white space. Trim that unused space after joining the sheets. Increasing overlap uses more paper, without enlarging the image.

Image quality

Making an image physically larger reduces its effective DPI. Below 100 DPI, close-up text or details may look soft. The tool warns you, but does not invent detail or perform AI enlargement.