Nothing is uploaded. When you drop or pick a PDF, the tool reads it locally with the browser’s File API, renders its pages in memory with the pdf.js engine (whose worker is bundled locally, not fetched from a CDN) and rebuilds the compressed file with the pdf-lib library on your own machine. There is no server, upload, database, API or tracking in the processing path, so certificates, ID scans, mark sheets and private paperwork you are shrinking for a form stay on your device; the tool works offline once cached, and closing the tab discards everything. The only optional network use anywhere on the site is consent-gated advertising, which never sees your files.

It depends on what is inside the PDF, but 400 KB is a comfortably roomy target. The real savings come from re-encoding images, so a text-only or lightly formatted document falls well under 400 KB, and a typical certificate, mark-sheet or multi-page scan reaches it with legibility to spare. A long scan full of photographs still has a floor: past a certain point the pages would turn to mush, so the tool stops at the smallest readable size it can reach and reports that figure with a note rather than producing an unusable file. If you are stuck above 400 KB, enabling grayscale, removing pages you do not need, or splitting the document first will usually get you under the cap. The three variants let you take the Smallest one when you need every byte, or Best Quality when 400 KB is comfortably met.

Reaching a target uses lossy compression — images are down-sampled to a lower DPI and re-encoded as JPEG, which is how a large file becomes small. But 400 KB leaves far more room than 200 KB or 300 KB, so the sweep can stop early: scans usually stay sharp, and a lighter document may meet 400 KB without whole pages being rebuilt as images, which keeps its text selectable and searchable. Three variants are generated in one pass (Best Quality, Balanced and Smallest) and the tool defaults to the best one that still fits. One honest note: when a heavier document does need its pages rasterised to reach 400 KB, the text on those pages becomes part of the picture and is no longer selectable; if you need the text to stay live on such a file, choose a larger target or a lossless PDF tool. For most certificates and forms headed to an upload box, the 400 KB result stays perfectly legible.

Some government, exam, pension and admissions portals permit uploaded documents up to 400 KB, and many others cap at 500 KB — where upload guidance suggests aiming around 80% of the limit for a safety margin, so a 400 KB file gives useful headroom under a 500 KB field and avoids last-minute rejections. Compared with a 300 KB, 200 KB or 100 KB target, 400 KB lets the tool compress more gently, so scanned certificates and mark sheets keep more of their detail while still clearing the field. This tool is built for exactly that: the target is fixed at 400 KB so you can drop the PDF and download a version that clears the limit, without hunting through compression settings. If the portal wants a smaller file, sibling tools target 300 KB, 250 KB, 200 KB and below, and larger presets (500 KB, 1 MB and up) exist for looser limits.