Resize PDF to A5 converts every page of a document to A5, 148 by 210 mm, or 419.53 by 595.28 points in PDF terms. A5 is half of A4, the size of a typical paperback notebook, and it is the format used for booklets, planners, zines, small manuals, recipe cards and many self published books. Because all A sizes share the same proportions, an A4 page shrinks to about 70% and fills the A5 page with the layout untouched, leaving at most a millimetre of white at the sides. That is the step print services and print on demand platforms ask for when the interior file must already be A5, and it also makes A4 documents easier to read on a phone or a small e-reader. Fit to size scales every page to fill A5. Add white space only reduces pages that are larger than A5 and centres smaller ones at their original size, so a set of mixed inserts ends up on one page size without enlarging small items. Landscape pages can become A5 landscape, 210 by 148 mm. Text and vector graphics stay sharp at the smaller size, though body text set at 11 pt on A4 becomes roughly 8 pt on A5, so check the preview for readability before printing. Note that this produces one A5 page per original page; printing two A5 pages side by side on one A4 sheet is a separate imposition step done in the print dialog or by the printer. The file is processed in your browser and never uploaded. To go back up a size use Resize All PDF Pages to A4, and to reduce the file size afterwards use PDF Compressor.
The reason inconsistent page sizes are so annoying is that nothing warns you about them. A PDF viewer happily renders a file where page 4 is Letter and the rest are A4, and the difference is only 6 mm wider and 13 mm shorter, which is invisible on screen. The cost arrives at the end of the process: a print driver set to "actual size" produces pages with different margins, a duplex job feeds wrongly, a submission portal that validates page dimensions rejects the upload, or a reader sees the page jump as it scrolls. Most PDF services offer resize, crop and compress as separate operations and leave the diagnosis to the user, which is the hard part. This tool inverts that order. It measures first, tells you in plain language what it found, and offers one button that applies the obvious fix. Only if you want something else do the other options matter. A few practical notes. Scanned pages rarely match a standard format to the decimal, so sizes within roughly 0.7 mm of each other are treated as the same format rather than reported as separate groups; this keeps the summary honest instead of listing twenty near-identical sizes. Pages carrying a rotation flag are measured as the reader sees them, so a portrait page rotated 90 degrees is correctly grouped with the landscape pages. The output is a standard PDF that opens in Acrobat, Preview, Chrome and any mobile reader, with text, fonts, links and images preserved. If your real problem is uneven white margins rather than uneven page sizes, PDF Crop & Resize is the better tool, since it trims the margins directly. If pages came from several source files, run PDF Merger first and normalise the combined document in one pass, and use PDF Page Count Checker to confirm the final file has every page you expect.