The reason this comes up is a geographic accident. The United States and Canada use Letter, at 8.5 by 11 inches. Almost everywhere else uses A4, which is slightly narrower and slightly taller. Documents crossing between the two print with wrong margins or a clipped edge, and print shops and submission systems often reject the mismatch outright.
Fitted and centred, never cropped
Each page is scaled to fit inside the new size, keeping its proportions, and centred. Nothing is cut off and nothing is stretched. Because A4 and Letter have different shapes, that leaves a small band of white space — a little at the sides going to A4, a little at the top and bottom going to Letter. That is unavoidable: two rectangles of different proportions cannot both be filled exactly without distorting or cropping.
The alternative approaches are worse. Stretching to fill makes text subtly wrong-shaped, and cropping to fill silently removes content from the edges, which is how page numbers and footers disappear.
What this changes about the file
This is the one page operation here that is not a straight copy. Each page is embedded into a new page of the target size, which preserves text as text and vectors as vectors — nothing is rasterised — but it does add a layer of nesting to the document structure.
- Text stays selectable and searchable, and images keep their original data.
- Links and annotations do not survive the re-embedding, since their coordinates refer to the old page.
- Form fields lose their interactivity.
- The file grows slightly from the added structure, though not meaningfully.
Mixed sizes get normalised
A document assembled from several sources often has pages of different sizes — a Letter cover on an A4 report, or a scanned page slightly off-standard. Running it through here makes every page identical, which is what a print shop needs and what stops a printer prompting about paper size partway through a job.
When not to use this
If the pages are already the right size and simply have margins you dislike, this will not help — use the margin trimmer instead, which changes the visible area without rescaling anything. And if the document is going to stay on screen, page size is largely irrelevant; it only matters when paper is involved.
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Check the result before printing a hundred copies
A page fitted from Letter to A4 is scaled to about 97% of its original size, which is imperceptible on body text but does shift anything positioned to an exact measurement. If the document is a form with fields meant to line up with pre-printed stationery, or artwork with a bleed, verify one sheet before committing to the full run.