.docx is a ZIP of XML (WordprocessingML). A script can unzip it, turn paragraphs into HTML, and paint that HTML to a canvas. That is the whole pipeline here: mammoth → HTML → html2canvas → JPEG.
.doc is a different, older binary format. Parsing it properly means a full Word engine. We do not ship one, and we do not upload the file to a server that has one.
If you only have .doc: open it in Word, LibreOffice or Google Docs and export .docx. Then convert.
Privacy
The document never leaves the device. Close the tab and the layout is gone. We still show ads and analytics on the page itself, which is separate from your file.