Makes a PDF smaller in the browser: the file is rewritten from its page tree so unreachable objects disappear, streams are re-deflated at maximum effort, objects that are byte-identical are merged, and non-stream objects are packed into compressed object streams with a cross-reference stream (PDF 1.5). Text, vectors and fonts are preserved exactly, and JPEG photos are never re-encoded. Turn on downsampling to re-sample large rasters — that part is not reversible. If the rewrite ends up larger, the original file is returned unchanged.