What this does, stated plainly
It reads the text and structure from your Word file and lays it out cleanly on A4: headings at heading sizes, bullets indented, paragraphs wrapped against the real measured width of the font.
It does not reproduce your document's exact visual design. Custom fonts, images, tables with merged cells, text boxes, columns and headers are not carried. That would require a Word rendering engine, which does not exist in a browser.
So this is the right tool when the content matters — a letter, a report, a CV draft, notes — and the wrong one when the design matters. For a designed document, Word's own Save as PDF is better, and it is also local.