Compress a PDF, see the difference.

No. It is compressed in this tab with your browser, so it never leaves your computer. That matters for contracts, statements and anything private.

It depends on what is inside. PDFs of scans, photos and slides with pictures often shrink a lot, because their pictures hold far more pixels than a page needs. A PDF of plain text is already small and may barely change; the page tells you when that happens.

Yes, at Light, Recommended and Strong. Only the pictures inside are changed; text, lines and fonts stay as they were, so you can still select and search the text. Smallest is the exception: it turns every page into a picture.

For scans, where every page is a picture anyway, or when a file must get under a size limit and selectable text does not matter. To read the words from such a page later, use image to text.

Pictures that are already small, drawings and screenshots with few colours (they would get blurry edges as JPEG), and pictures stored in print colours (CMYK) or rare formats. A picture is only replaced when the new one is clearly smaller.

It drops leftovers that nothing in the file uses any more, which edited PDFs often carry, compresses page text stored without compression, and packs the file more tightly. Fonts are kept whole.

A PDF locked with a password cannot be read here. Open it, save a copy without the password, then compress that copy.