Reduce PDF Size in KB — Free, Fast & Private
"Reduce PDF size" is rarely a wish for a smaller file in general. It is a number on a form: a box that says the PDF may be so many kilobytes and refuses anything larger. This page is the starting point for that job. The tool above has one control that matters, the target, and it offers the four limits that forms most often state. Pick yours, drop the PDF in, and the finished file is measured against it.
Which limit to pick
Use the limit your form prints, or the nearest one below it, never the nearest one above. Each target has its own page with the detail for that size: 100 KB is the tightest and suits one or two scanned pages; 200 KB is the common middle; 500 KB suits a short multi-page certificate; and 1 MB is for larger scanned bundles. If your form states no limit, leave the target off, or use the plain PDF compressor, and keep full quality.
What the tool does to reach it
Each page of the PDF is rendered in the browser and re-encoded as a JPEG image, keeping the original page size, and the document is rebuilt around those images. With a target set, a byte search chooses the JPEG quality page by page, and if quality alone cannot get under the budget the page image is also reduced in pixel dimensions, in steps. The finished document is then measured. If it is at or under the target you see that size; if not, the tool says the target was not reached and shows the smallest size it managed.
That method suits scans and photographs of paper, where the page already is a picture. It is the wrong tool for a PDF exported from a word processor, where every page becomes an image and the selectable text is lost; the tool warns you when it finds live text on the first pages.
When the PDF starts as photographs
If what you have is a set of photographs rather than a PDF, build the PDF to the size directly instead of compressing afterwards: JPG to PDF under 100 KB, 200 KB or 500 KB fits each image toward its share of the budget before it is embedded, which loses less than shrinking a finished PDF.
Decimal kilobytes
Targets are decimal: 100 KB is 100,000 bytes. A file that fits under that also fits a portal that counts 1,024-byte kilobytes, so the decimal reading is the safe one for either kind of form.