PDFs get big for a few simple reasons: scans and photos saved at full camera quality, the same image stored again on every page, and files written by programs that do not bother to tidy up. A big PDF is slow to send, bounces off email limits and gets refused by job, exam and government portals that cap uploads at a few hundred KB or a couple of MB. Compress PDF makes the file smaller so it goes through.
There are two levels. Strong is the one most people want. It keeps every page looking the same at normal zoom, but re-saves the photos and scans inside the PDF at a sensible size and quality, which is where almost all the weight is. Text, lines and vector drawings are not touched, so they stay sharp and searchable. Basic changes nothing you can see. It only rewrites the file more efficiently, removing unused parts and packing the rest tightly, which helps most with text-heavy documents.
How much you save depends on what is inside. A PDF made from phone photos or a scanner often shrinks by half or more, sometimes by ninety percent. A PDF that is mostly typed text was usually small to begin with and shrinks less. The tool never hands you a bigger file: if a file is already as compact as it can be, you get the original back, and the result screen tells you the size before and after.
Compression runs entirely in your browser, on your own device. Your files are not uploaded, which matters for salary slips, ID cards and medical reports, the very files people most often need to shrink for an upload form. You can add several PDFs at once and download them one by one or together as a ZIP. There is no sign-up and no watermark.
If a portal asks for an exact limit, such as under 100 KB or under 200 KB, use the exact-size compressor instead: you type the target and it searches for the best quality that still fits. A password-protected PDF has to be unlocked first. To make a long file smaller in another way, Remove pages takes out pages nobody needs.