File, Save As, PDF, from the same machine where the document looks right.
Jump to any page with a table, a formula, or the last page. Look for rewrapped lines and shifted page numbers.
Swap to a system font, or replace the fragile pages with images, then re-export.
The document references a typeface by name. If the machine doing the rendering does not have it, a substitute is used, and every character width changes. That is why a document looks perfect on your laptop and reflows the moment you convert it elsewhere: headings wrap, tables push a column onto the next line, page numbers shift.
Chinese characters are full width and Latin characters are not. If the substitute font has a different full-width advance, one character per line no longer fits, the paragraph rewraps, and everything below moves. Tables show it worst because column widths are fixed: the moment text gets wider it wraps into two lines and the row height doubles.
Export to PDF from Word on your own machine first — it uses the fonts you actually have. Font embedding is the second option, but Chinese typefaces add 10-20 MB and many commercial fonts forbid embedding outright. Online conversion is last, because the server only carries a handful of common fonts; if you must use it, substitute system fonts beforehand and turn the table-heavy or formula-heavy pages into images so the layout is locked.
Because the font was never in the file, only referenced by name.
It does, but Chinese fonts add 10-20 MB and some licences prohibit embedding.
The server has a limited font set, usually a few common families, so substitution is more likely.