A4 documents on American paper
Letter, 8.5 × 11 inches (21.6 × 27.9 cm), is the standard paper size in the United States, Canada, Mexico and several other countries in the Americas. Most of the rest of the world uses A4, which is slightly narrower and noticeably taller. When a document made in Europe, India or Australia arrives as an A4 PDF, many printers in the US either ask for A4 paper, which nobody has in the tray, or cut off the bottom of every page.
Converting the PDF to Letter fixes that before you print. It is also what some US portals, schools, courts and agencies expect when they ask for documents on Letter-size pages, and it keeps a set of files consistent when you merge papers from different countries.
Typical cases: a thesis or a CV written in A4 that you must submit to a US university or employer, a manual from a European supplier for your office, an Indian or British certificate for a US application, or a set of scans in mixed sizes that you want to print as one tidy stack.
Scaled to fit, never cut
Each page is scaled as a whole to fit the Letter page, keeping its proportions, and centered. Nothing is cropped and nothing is stretched. Because A4 is a little taller than Letter in proportion, an A4 page shrinks slightly and you will see narrow white bands at the sides; that is the price of keeping the content intact.
Landscape pages stay landscape, on a Letter page turned sideways, so wide tables and charts keep their layout. The text is not converted to an image: it stays sharp, searchable and selectable.
What to check before printing
Print the new PDF at 100% or “Actual size”. If your print dialog is set to “Fit to page”, it will probably make no difference now, but turning it off avoids a second resize. If you added the extra margin, check that it suits your printer, especially if it cannot print close to the edge.
Links, form fields and annotations from the original are not kept. If the PDF is a form you need to fill in, fill it first, then flatten it with “Flatten PDF”, and then change the page size.
Private and free
The new pages are built in your browser. Your PDF is not uploaded to a server and no copy is kept, which matters for contracts, transcripts and immigration paperwork. It is free, with no account, no watermark and no limit on use.
For other sizes, such as A4, Legal or a custom size, use “Resize PDF”. The opposite conversion, Letter to A4, has its own page too.