An estimate and a rendered document answer different questions
The estimate starts with an assumption of 500 words on a US Letter page with one-inch margins, 12-point Times Roman and single spacing. It scales that capacity by usable page area, the square of the font-size ratio, line spacing and a simple font factor. It is a planning model, not a measurement of your text.
In that baseline layout, 1,000 words gives approximately two pages. Double spacing halves the assumed capacity and gives approximately four pages. In reverse, a four-page budget at 250 words per page gives an estimated 1,000 words. A4 paper has a different usable area, so the result changes. Short paragraphs, headings, bullet lists and long words can shift the real page count further.
Generating a PDF is a separate operation. The renderer measures the actual text with the chosen standard PDF font, wraps it into lines and adds pages as needed. The displayed actual page count belongs to that generated PDF only. It does not promise identical pagination in Microsoft Word, Google Docs or another editor.
The built-in preview fonts have a limited character set. Unsupported characters cause an error instead of being silently removed. For other scripts, continue to the full Text to PDF workflow and check whether a suitable font is available. If the PDF preview is unavailable on your phone, download the file and open it in your normal PDF viewer.