Smaller PDFs, same text
Most of the size of a PDF usually comes from the photos and scans inside it. The compressor finds these images, scales down the ones that are larger than needed for screen and office printing, and saves them again as efficient JPEG. Text, vector graphics, links, bookmarks and form fields are not touched, so the document stays sharp and searchable.
If an image would not get smaller, the original is kept. If the whole file would not get smaller, you get the original file back instead of a bigger one.
Private by design
Contracts, bank statements, medical letters and CVs often need compressing, and they are exactly the files you should not upload to an unknown server. Here the work happens in your browser. Nothing is sent anywhere, which also means there is no file size limit and no waiting in a queue.
For scanned documents that are one big image per page, choose Pages to images. Each page is rendered and saved as a compact JPEG, which can shrink large scans dramatically; text in the scan stays readable but is no longer selectable.