A4, Letter and Legal: PDF page sizes and the scaling trap when you print

A PDF page carries a media box — the physical sheet — and optionally a crop box, which defines the region a viewer or printer should display. Crop the content by shrinking the crop box and a reader will show the document as trimmed while the underlying sheet stays the same size. That is how a scan that was captured with a black border along one edge can be made to look clean without any quality loss.

The distinction matters when a document is sent to a print shop. The media box decides what paper is used, so a Letter-sized file printed onto A4 paper needs scaling — and scaling a form that is going to be filled in by hand at a counter is exactly what produces misaligned boxes.

Print dialogues offer Fit to page, Shrink to fit, and Actual size, and the first two are the default on most systems. Fit to page enlarges small content to fill the sheet or shrinks large content to fit inside the printable area, and the resulting scale is rarely a round number.

For a document you are reading, that is fine. For a form, a payslip, or anything with dimensions on it, it is not: a 100% A4 document printed at 94% produces boxes that are 6% smaller than the form you were told to fill, and photograph boxes that no longer match the specification. Set the dialogue to Actual size or 100% whenever a document is a form.

The other trap is the printable area. Consumer printers cannot print to the outermost two to five millimetres of a sheet. A design that runs text to the very edge will be clipped when printed at actual size, and fitted when printed with the default setting — which is why the same file can be cut off on one printer and slightly small on another.

When a document must be A4 and is not, you have two options and they are not equivalent. Padding adds white space around the existing content so that the page becomes the right dimensions without altering anything inside it — safe, and the right default for scans and photographs.

Scaling changes the physical size of the content, so a form designed for Letter becomes slightly narrower when pushed onto A4. It preserves the layout of the page while changing the size of everything on it, which is why scaling is acceptable for images and rarely acceptable for forms.

Converting an image to a PDF with A4 pages, which our converter offers, is the padding case: your photograph is placed on a full A4 sheet with the rest left white. If you want the page to be exactly the size of the image instead, the fit-to-image option does that, and produces a smaller file as a side effect.

A flatbed scanner set to Letter produces 216 × 279 mm pages even when scanning an A4 sheet, which leaves a white strip along one edge and crops nothing. A phone photograph converted to a PDF with fit-to-image pages produces a page the size of the photograph, which could be anything from a credit card to a poster.

ADF scanners, the kind that feed pages in, add a few millimetres of skew and often a black edge on one side. Neither is a defect worth fixing, but both explain why a bundle assembled from several sources will look inconsistent page to page, and why normalising to A4 is worth doing before printing and posting anything.

Duplex printing, or printing on both sides of a sheet, is where page size and printer behaviour collide. If the printer flips on the long edge and your document is a mix of portrait and landscape pages, every landscape page will come out upside down relative to its neighbours — harmless for reading, awkward for a bundle that will be signed and filed.

The margin matters for a different reason. Forms usually reserve the outer twelve to fifteen millimetres for the portal reference number, the signature box or the office use only panel, and a printer that quietly fits the page to its own printable area shrinks those regions first. When a form says to leave the top right corner blank, that instruction is about the physical sheet, so print at actual size and keep the paper orientation as the form expects.

For a bundle that will be photocopied or scanned again by someone else, leave at least a ten millimetre clear margin on every edge. Cheap scanners crop a little, and a page number that sits on the boundary is the first thing to disappear.

Every PDF reader shows page dimensions somewhere — usually in document properties, or in the status bar at the bottom of the window. Scroll to a page and read the size before you send a document to a printer, a print shop or an upload portal, and confirm the scale setting in the print dialogue the next time you print a form. Those two checks take about fifteen seconds and eliminate the entire class of problem described here.