Word files get large because photos and screenshots are stored inside the .docx at full size. The text itself is usually only a few kilobytes. Shrinking those images is what makes the document small enough to email or upload.
This tool opens the file in your browser, compresses the images, and rebuilds the Word document. Your file is never uploaded. Text, headings, and layout stay in the document. Pictures may look slightly softer at the Strong setting.