Word already knows what a heading is
Converting a PDF means guessing at structure that was thrown away when the file was made. A .docx is the opposite. It carries its own outline. When you styled a line as Heading 1, Word wrote that down, and this converter simply reads it.
That single fact is why a Word to EPUB conversion is reliable in a way that a PDF conversion never is. Your chapter breaks are your chapter breaks. Your bulleted list stays a list rather than becoming a paragraph starting with a bullet character. Your block quote stays a block quote.
Use Word's styles rather than making text big and bold by hand, and the ebook will come out the shape you wrote it.
What carries across and what does not
Headings, paragraphs, bold, italics, bulleted and numbered lists, block quotes, tables, hyperlinks, footnotes and embedded images all survive. Images are pulled out of the document and stored properly inside the EPUB rather than pasted in as base64, which keeps the file small enough to email.
Page breaks, headers, footers, page numbers, columns, text boxes and anything positioned by dragging it around the page do not survive, because an EPUB has no pages to put them on. Fonts do not carry across either. The reader picks the font, and that is the point of the format.
Track changes and comments are ignored. Accept or reject them before you convert, or they simply will not appear.
Before you send it to KDP, Kobo or Draft2Digital
Retailers reject malformed EPUBs and they are not always specific about why. The files this converter produces are built to the EPUB 3 specification and validate against epubcheck, the official validator published by the W3C, with no errors and no warnings.
A few things are still on you. Set the title and the author in the fields above rather than leaving the converter to read them from the filename. Delete the table of contents you typed by hand, because the EPUB builds a real navigable one and two of them is worse than none. Make sure every image has alternative text, since accessibility is now checked as well as validity.
A cover is not part of this conversion. Retailers want one supplied separately, and they all want it at a different size.
A manuscript is not a designed book
What comes out of this tool is your text, correctly structured, in a file any reader can open. It is not laid out. There is no cover, no drop cap, no chapter opener, no considered typography, because none of that was in the .docx to begin with.
That is fine for a proof copy, a beta reader, an internal draft or a lead magnet nobody will judge on its looks. It is not what you publish under your own name.