Reduce PDF size is the job people describe when an upload form rejects a file, or a mail server bounces an attachment. On this site it is the same tool as compress PDF. Each page is painted in the browser and saved as a JPEG inside a new PDF. That is why a scan of a paper form usually shrinks, and why a short digital letter sometimes grows. The form states the tradeoff in the box above the button: text will not be selectable, links inside the file disappear, and a number you type is a target rather than a contract.
Use "just make it smaller" when any saving will do. Use 200 KB or 100 KB when a form named a ceiling. The converter tries clearer settings first and stops when the file fits. If it cannot fit, it tells you the smallest size it reached. Download that file only if you can still read it. Keep the original either way. This page does not overwrite the file you selected.
The PDF is not uploaded. Drawing uses pdf.js and the new file is written with pdf-lib, both served from this website, and only after you press the button. Twenty-five pages is the cap. Longer documents need to be split before they will run here. Photo pages that started as JPG to PDF are the files this tool is good at. Several documents should be joined with merge PDF first if the limit applies to one attachment.
Open compress PDF to 200 KB or 100 KB if you want the target pre-selected. Both pages repeat the same warning. A file that is already under the target can still be redrawn if you press the button, and the redraw can be larger than a tiny text original. Check the two sizes in the note before you throw the first file away.