Shrink a PDF by re-encoding the images inside it. Phone scans store every page as a raw bitmap and usually come out 80-95% smaller; text, pages and annotations are carried through untouched. All processing happens in your browser.
Still sharp on screen and at A4 print size. A good default for scans.
Only images are re-encoded. Anything a JPEG cannot hold — transparency, stencil masks, colour spaces that would not survive the round trip — is left exactly as it was, as is any image that would come out larger. All processing happens in your browser — the file never leaves your device.