PDF compression levels explained: Low, Recommended, High

The three levels on Compress PDF are not “10% / 30% / 60% smaller.” They are two different algorithms and one milder setting of the first.

Low and Recommended — re-encode images, keep operators

Both walk image XObjects, decode, and write JPEG at a quality that is higher on Low and lower on Recommended. Content streams that show text (Tj, TJ, fonts, vector paths) are kept. You can still highlight a paragraph. Forms, if they were real AcroForm fields, have a better chance of surviving than they do on High. Recommended is the email default. Low is for a file that is only slightly over a cap and still needs to look like a print PDF.

High — flatten

High rasterizes each page to a bitmap and builds a new page whose only job is to paint that image. Text operators are gone. Search is gone. Form fields are gone. Size drops because you threw away structure. Use it when Recommended still misses a 5 MB portal and you do not need to copy text. Do not use it on a text-only 200 KB memo; you will replace vectors with a page-size JPEG and the file will grow.

Rule of thumb: if the page is a photograph of paper, all three levels are “how hard to JPEG it,” and High just does it as a whole page. If the page is born-digital text, Low/Recommended barely move the needle and High is vandalism.