What this actually does, in one paragraph
Compressing here renders every page to an image at a reduced resolution, re-encodes those images as JPEG at a chosen quality, and rebuilds the document from them at the original page dimensions. The result prints at the same size and looks right on screen. It is the same approach a photocopier takes, and for the documents people usually need to shrink — scans, brochures, anything photographic — it is dramatically effective. A 40 MB scanned contract routinely comes back under 3 MB.
The cost, stated plainly
A compressed page is a picture of a page. The text in it is no longer text: you cannot select it, you cannot search it, and a screen reader cannot read it. If the document was already a scan, you have lost nothing, because it was already a picture. If it was a real text document — an invoice exported from accounting software, a contract from a word processor — you have traded away something you may want back, and no setting here will return it.
That is why this page says it before you press the button rather than afterwards. If you need the text to stay searchable, compressing is the wrong tool: try splitting the document into the part you actually need to send, or check whether the size is coming from a handful of oversized images rather than the document as a whole.
Choosing a level
Light renders at roughly twice the page resolution and barely touches quality. Use it when the file has to shrink but will still be printed. Medium is the setting most people want: clearly smaller, still crisp on a screen, fine for email and document portals. Strong produces the smallest file and is honest about being soft when you zoom in — good for a receipt or a form upload with a tight limit, poor for anything detailed.
Because everything runs locally, trying all three costs nothing but a few seconds. Run one, look at the number in the readout, and run another if it is not enough.
When compression makes a file bigger
This surprises people, so the studio checks for it and tells you. A PDF that is mostly vector text is already extremely compact — the letters are stored as instructions, not pixels. Turning those instructions into images can easily produce a larger file than you started with. If that happens, the status line says so and recommends keeping your original. Your file on disk is untouched either way; the studio only ever hands you a new copy.