Find out what is making the file heavy
PDFs are containers. A ten-page text document is typically under 1 MB, while a single phone photo dropped into a page can be 6 MB on its own. If your file is large, scanned pages or screenshots are almost always the reason.
Open the file and scroll through it. Pages that look like photographs of paper are scans, and those are the pages worth compressing. Pages of selectable text are already small and compressing them gains you nothing.
Split before you compress
If the recipient only needs the signature page or one appendix, sending the whole document is wasted effort. Splitting out the pages that matter is the single biggest size reduction available, and it costs no quality at all.
Split the file first, check that the extracted pages are the right ones, then decide whether the result still needs compressing.
Compress the images, not the text
Good PDF compression re-encodes the images inside the file and leaves the text layer untouched. That keeps headings crisp and search still working, while the file gets dramatically lighter.
For scanned documents, exporting the pages as images, compressing them, and rebuilding the PDF is often the most effective route. Aim for roughly 150 DPI for anything that will be read on screen, and 300 DPI only when the document will be printed.
Why doing this in the browser matters
Contracts, invoices, medical letters and ID scans are exactly the documents you should not be uploading to an unknown server. Browser-based tools read the file directly from your disk, do the work in the page and hand you the result — the document never travels anywhere.
It is also faster. There is no upload wait, no queue and no download step; on a normal laptop a 40 MB file is processed in a few seconds.