Why page size mismatches cause so much trouble
A4 is 210 × 297 mm and is used almost everywhere in India, Europe and most of Asia. Letter is 216 × 279 mm and is standard in the United States and Canada. The difference is only 8 mm in one direction and 18 mm in the other, but it is enough to ruin a print job.
Print a Letter document on A4 paper without scaling and you lose a thin strip along one edge — often a page number or the end of a line. Print A4 on Letter paper and the same thing happens in the other direction. Combine pages of both sizes into one document and the printer has to pick a single size, so half the pages get scaled or clipped.
Scanners add to the problem: a scanner set to Letter will happily scan an A4 page and clip the bottom, or scan it with a white gap that looks like a design error. And a phone photograph has no paper size at all — its page is whatever shape the camera produced.
Resizing puts every page on the same, correct sheet. That is what makes a document printable, submittable and consistent.
Fit, stretch, or keep the original size
Fit (no cutting) scales the original page down proportionally until it fits inside the new sheet, and centres it. Nothing is lost, proportions are preserved, and if the aspect ratios differ you get a small white border. This is the safe default and the right choice for almost everything.
Stretch to fill scales width and height independently so the content fills the new page exactly, with no borders. It looks perfect when the aspect ratios are close — Letter to A4, for example — and visibly wrong when they are not, because text and images become noticeably stretched.
Keep the original size places the original page at its true dimensions on the new sheet, centred, without scaling. It is only scaled down if it would not otherwise fit. This is the right choice when you are adding a margin around content rather than converting a format — for example, putting a small receipt or a label onto a full A4 page.
The margin slider adds uniform white space around the content on top of whichever mode you choose, which is useful for documents that will be hole-punched or bound.
The resize you probably need: Letter to A4
Most documents travelling between the US and the rest of the world need this one conversion. Letter is 94% of A4's width and 94% of its height, so the content needs scaling by 94% to fit comfortably on A4 — a change you will not notice.
Choose A4, set fit mode to “Fit (no cutting)”, and leave the margin at 0. The result is a document with every page on A4, content unchanged apart from a very slight reduction in size, and a 4 mm band of white space at one edge. It prints cleanly on any Indian printer and scans without clipping.
The reverse — A4 to Letter — works the same way. If the document will be printed on Letter paper and you want it to fill the sheet rather than sit inside a border, use “Stretch to fill”; at a 6% difference the distortion is imperceptible in text and acceptable even in most graphics.
Good reasons to resize a PDF
Standardising a mixed document: a report assembled from Letter and A4 parts becomes consistent, which is what makes it look professionally put together.
Putting a small item on a standard sheet: a 10 × 15 cm photograph, a receipt photo or a scanned label becomes a full A4 page, ready to file or paste.
Meeting a portal’s page-size requirement: some upload systems specify A4 and reject anything else. Resize before uploading rather than after a rejection.
Printing on a specific format: if your printer only holds A5 or you need an A3 poster version of an A4 document, resizing is a one-step conversion.
Adding a margin for binding: a document destined for a comb bind or a ring binder benefits from a consistent extra margin, which the margin slider provides on every page at once.