PDFs often end up with pages of different sizes. A scanner app saves receipts as tall strips, a US form arrives in Letter while everything else is A4, slides from a presentation are wide, and a merged file mixes all of them. Printers then shrink some pages, cut off others, or ask for paper you don’t have, and an upload portal may reject the file outright because it only accepts A4. Resize PDF puts every page on the same paper size in one go.
Choose the paper size: A4, the standard in most of the world, Letter for the US and Canada, Legal, A3 or A5. By default each page keeps its own orientation, so a wide slide lands on a landscape page and a normal page on a portrait one. You can also force every page to portrait or to landscape, which is handy when the file must print in one direction.
The content of each page is scaled to fit and centred, so nothing is cut off. Where the proportions differ, for example a Letter page placed on A4, a thin white margin appears on two sides. Small pages such as receipts or ID card scans are enlarged to fill the new page; turn off Enlarge small pages to keep them at their real size in the middle of the sheet instead, which is better when the exact print size matters.
Text stays real text and images keep their quality, because the pages are rescaled, not turned into pictures. Links and comments move with the content. All of this happens in your browser on this device, so the file is never uploaded, and it works the same on a phone, tablet or computer.
Resizing changes the paper size, not the file size. If the result is too big for an email or a form, run it through Compress PDF afterwards. To remove white borders or cut away part of a page, use Crop PDF, and to turn sideways pages the right way up, use Rotate PDF.