Compress a PDF without uploading it

People compress a PDF because something refuses to accept it: an email attachment cap, a tax portal, a visa application, an insurance claim form. Those are exactly the documents you would not choose to hand to an unknown intermediary, and the size limit is a technical annoyance rather than a reason to change who holds your paperwork.

Compression here targets the images. A scanned document is a stack of pictures and shrinks a great deal; a contract exported from a word processor is mostly fonts and structure, so there is very little to squeeze and the result may be close to the size you started with. That is the file being honest with you, not the tool failing.

Judge the output at full zoom before you send it, not at page-fit. A compressed scan of an identity document is worthless if the number is no longer legible, and small print in a footer is the first thing to blur. If it went too far, start again from the original rather than compressing the compressed copy.

Verifying it is straightforward: open the network panel, drop the file in, compress, and watch that nothing carrying the document goes out. Disconnecting first works too, and it is the more convincing test because it removes the possibility of missing a request.