Sharing a Word document as a PDF usually means uploading it to a converter site or opening desktop Word. This reads the .docx file locally, reconstructs it as HTML, and renders that as a paginated PDF, all in your browser tab. Handy when you need a quick PDF copy of a resume, report, or letter without installing anything or trusting a third-party upload.
From a reflowing document to a fixed one
A DOCX does not describe pages. It describes content and intent: this paragraph has this style, this table has these columns, this image floats here relative to that text. Where the page breaks fall is computed at the moment the document is opened, by whatever is opening it, which is why the same file can paginate differently on two machines.
A PDF is the opposite. It records final positions: this glyph, this font, at these coordinates on a page of this size. Converting means performing that layout once and freezing the result, which is precisely why you would send a PDF to a printer, an employer or a client rather than the DOCX.
Fonts and complex layout
Because the layout is computed during conversion, it depends on the fonts available at that moment. A document set in a licensed corporate typeface will be laid out with a substitute if that font is not present, and substitutes have different letter widths, so line breaks and page breaks move. The safest sources are documents using widely available fonts.
Faithfulness also drops as a document gets more elaborate. Straightforward reports, letters and CVs convert cleanly. Heavily designed documents leaning on floating text boxes, multi-column layouts, footnotes, tracked changes or embedded charts are where a converted result is most likely to differ from what Word showed you. Open the output before sending it, particularly if it is going to a printer.