What is actually large inside a PDF
A PDF is a container, and the reason one is 40 MB is almost never the text. Text is stored as glyph references and font programs, and even a long document rarely spends more than a few hundred kilobytes on them. The size is in the images: scanned pages, photographs, screenshots and logos, each stored at whatever resolution it was placed at, which for a phone-scanned document is often far higher than anything the page can display.
That leads to the single most useful distinction when shrinking a PDF, and the one most tools hide from you. Downsampling and recompressing the images inside a PDF keeps the document a document: the text stays text, remains selectable, remains searchable, and still works with a screen reader. Rasterising the pages, which is what "maximum compression" usually means, converts every page to a flat picture. The file gets much smaller and the text stops being text.
The difference matters most in exactly the situations where people compress PDFs. A visa application, a tender submission or a court filing may be checked by software that reads the text. A rasterised page passes a size check and fails everything else, silently, and you find out later. So this tool downsamples images first and reports what it did, and if hitting your target genuinely requires rasterising, it says so before it does it rather than after.
The other lever is what resolution the images need to be. A page destined for a screen or an ordinary office printer gains nothing from 600 dots per inch of scan data. 150 DPI is generally indistinguishable on screen, and often halves or quarters the file on its own. Where a scan is a photograph of a white page, converting it to greyscale removes two thirds of the colour data at essentially no cost to legibility.
As with everything here, the work happens in your browser. That matters more for PDFs than for most formats, because the documents people need to compress are disproportionately the ones they should not be uploading: bank statements, medical records, identity documents, contracts under NDA.