What maximum compression actually does
This page is Compress PDF with the Extreme level preselected. It's the most aggressive of the engine's three settings: each photo inside the PDF is shrunk until its longest side is at most 1100 pixels and re-saved as JPEG at 50% quality. For comparison, the Recommended level keeps 1700 pixels at 70%, and Low keeps 2400 pixels at 85%.
What never changes, at any level, is digital text and vector graphics. They stay sharp, and the text can still be selected, searched and copied. Embedded page thumbnails and XMP metadata are also stripped, and they sometimes weigh more than you'd expect.
When extreme compression makes sense
Use it when file size matters more than image quality. Think of a long scanned file you need to email under an attachment limit, a stack of records for a slow upload portal, a product catalog that will only be viewed on screen, or a PDF you want to keep on your phone without eating storage.
It works best on PDFs made of photos or scans, because that's where almost all the weight is. A 300 DPI scan of a Letter or A4 page is more than 2500 pixels tall; bringing it down to 1100 shrinks the file a lot, and for most scanned text documents the lettering is still readable on screen.
What you give up, and when to skip it
To be straight with you: at 50% quality and 1100 pixels, photos lose detail. You'll notice it in tiny print, fine stamps and signatures, drawings with thin lines, and photos someone will zoom into. If the document will be printed, is a contract with small print, or has images that must stay crisp, try the Recommended level first.
Compression can't be undone, so always keep the original. If the PDF is text only, savings will be small, because its weight is in the fonts, not in images. Images with transparency, 1-bit, indexed, CMYK, JPX or JBIG2 images are left untouched so they don't get damaged, so a PDF full of them may barely shrink.
Maximum vs. a target size
Maximum and Compress to X KB are not the same thing. Extreme compression always applies the same settings and gives you whatever size results. If a form demands a specific limit, such as 200 KB or 1 MB, the target-size mode is the better fit: you type the number and the engine lowers quality step by step until the file fits, without going further than needed.
A trick that helps: if the file is still too big after extreme compression, look for blank or duplicate pages and drop them with Remove pages. Every scanned page you remove is weight that disappears.