PDF Compressor
Most of a PDF is embedded images, so that is where this spends its effort: they get decoded, re-encoded with MozJPEG or AVIF at a resolution matched to how large they actually appear on the page, and scored against the original. The text layer is left alone, so the result stays selectable and searchable. Runs entirely in this tab
or click to choose — including password-protected files you already have the password for
Why not just rasterise everything?
Flattening each page to a JPEG is the oldest trick in this category and it does produce a small file — by throwing away the text layer, the vector graphics and every hyperlink. The result is not searchable, cannot be copied from, and looks soft the moment anyone zooms in. That is why it is the last option here and off by default.
Why is Zopfli so slow?
It searches for a better deflate encoding for roughly a hundred times longer than zlib does, and the payoff is typically 3–8%. On a 20 MB document that is minutes of work for a megabyte. It is on by default because the payoff is real, but "Balanced" effort caps its iterations — switch to Maximum only if you are prepared to wait.