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“Compress PDF” is not one operation. A PDF's size can come from very different places, and the fix for one does nothing for the others — which is why a single magic button so often returns a file the same size as the one you gave it, or a file whose text has silently stopped being searchable.

Where the bytes actually are

In a scanned document, essentially all of them are in page images. In a report exported from a word processor, they are usually in embedded font subsets and in structural overhead. In a file that has been edited and re-saved a dozen times, a surprising share is in material that is no longer referenced at all: PDF supports incremental saving, where each edit appends to the file rather than rewriting it, so previous revisions can still be sitting in the bytes.

The lossless rebuild

This copies every page into a brand-new document. Copying walks the object graph from the page tree, so only what is actually reachable comes across — old revisions, orphaned fonts, images belonging to deleted pages and abandoned annotations are simply not visited. The result is then written with object streams, which pack the many small dictionaries describing pages, fonts and links into compressed streams rather than plain text.

Nothing is re-encoded. Text stays selectable and searchable, vector art stays vector, images are untouched. On a heavily edited file this can remove a large fraction; on a file that was written cleanly in the first place it removes almost nothing, and this page reports the real number rather than a flattering one.

The rasterising pass

This renders each page with pdf.js — Mozilla's engine, the one Firefox uses — and rebuilds the document from those images as JPEGs. For a scan the difference is enormous: a 200 MB scan at 600 DPI becomes a few megabytes at 120 DPI with no visible loss, because the original resolution was far beyond what the content justified.

It is also destructive in a way worth stating plainly. The text layer is gone. The document is no longer searchable, no longer selectable, no longer readable by a screen reader, and links stop working. For a scan there was no text layer to lose and this costs nothing. For a text document it turns a good file into a bad one, which is why it is not the default.

Choosing a resolution

120 DPI is legible on screen and prints acceptably. 150 DPI is comfortable for printing. 200 and above rarely earns its size for a scan of ordinary text. The JPEG quality matters just as much: 0.6 is the sweet spot for scanned text, where artefacts sit in the paper texture and nobody notices them.

What this tool will not claim

It will not promise a percentage. Any site advertising “up to 90% smaller” is quoting the rasterising case for a high-resolution scan and applying it to everything. If your PDF is already efficient, the honest answer is that it cannot get much smaller without losing something, and that is what you will be told here.

Everything happens in this tab

The rebuild runs in a Web Worker with pdf-lib; the rendering uses pdf.js, also in your browser. The document is never uploaded, and the page's Content-Security-Policy sets connect-src 'self', so an upload would be blocked by the browser rather than merely not attempted. For contracts, payslips and medical letters — the documents people most often need to shrink for an upload form — that is the only arrangement that makes sense.