Compress PDF
Honest limit, please read: A browser cannot recompress the images inside a PDF as well as a desktop program can. This tool re-saves your PDF in a compact form in your browser. It sometimes saves a little, and sometimes saves almost nothing. It does not promise a fixed percentage saving. The page always shows the original and new size, so you can judge for yourself. Privacy note: Your PDF is not uploaded to a server, stored or sold. Works best for PDFs under 10 MB.
How to actually make a PDF smaller
Most of a PDF's size comes from the images inside it. A browser re-save can remove some unused data, but it cannot shrink those images much without turning each page into a picture, which would also destroy selectable text. So, if this tool only saves a little, the practical next step is to go back to the source: use smaller images, or fewer pages, before you make the PDF, or use the 
Image Compressor and 
JPG to PDF tools to rebuild it from smaller images. If the new file here is larger than the original, do not use it — keep your original file. That result is shown honestly on this page, not hidden.
FAQs
Why did my PDF not get much smaller? Because its size is mostly in its images, and a browser re-save does not recompress those images. This page reports the real sizes so you are not misled.
Is my PDF uploaded? No. Compression happens in your browser.
Should I download the new file if it is larger? No. Keep your original file in that case.