How this compressor works
Each page is redrawn at a reduced resolution and stored as a JPEG inside a new PDF. The tool then tries a range of quality settings, measuring the file after each attempt, until it finds the highest quality that still fits under your target. If even the lowest quality is too large, it drops the resolution and tries again.
What you give up
Because pages become images, text in the compressed file is no longer selectable or searchable, and it will not be read by screen readers. For a scanned document or a photo-heavy brochure that costs you nothing. For a contract someone needs to search, or a form someone needs to fill in, it matters — in that case use a gentler target and check the result before sending it.
Why nothing is uploaded
The whole process runs in JavaScript on this page. Your file is read into memory by your own browser, redrawn by your own graphics hardware, and saved back to your own disk. No copy is sent anywhere, there is no server holding your document, and nothing needs deleting afterwards. You can disconnect from the internet after the page loads and it will still work.
Why a file can't always reach the target
A PDF has a floor. Page count, page dimensions and the structure of the document itself all take up space before any image data is added. A 200-page document will not fit into 200 KB at any quality. When a target is unreachable the tool compresses as far as it sensibly can and tells you the size it landed on.
Is there a file size limit?
No hard limit, but your browser's memory is the practical ceiling. Very large documents — hundreds of pages, or pages at poster dimensions — are processed at a lower resolution automatically to avoid running out of memory. On a phone, expect slower going than on a laptop.