What this actually does
This re-saves your PDF using more efficient internal object encoding (object streams), which removes redundant structural data — genuinely useful for PDFs with many form fields, annotations, or that have been edited repeatedly. It does not recompress embedded photos or scanned images, which is usually where most of a PDF's size actually comes from. If your PDF is mostly scanned pages or high-resolution images, expect a small reduction here, not a dramatic one — for that, the underlying images would need to be recompressed individually, which a browser-based tool like this can't do reliably.
related tools
Need to combine or split files instead? Try Merge PDF or Split PDF.