What ODT files are
ODT (OpenDocument Text) is the standard document format of LibreOffice Writer and Apache OpenOffice. It is an open international standard used by many schools, universities, government offices and Linux users. People without LibreOffice often cannot open ODT files properly, especially on phones.
What happens when you convert
The ODT file is unpacked in your browser and its headings, paragraphs, lists, tables and embedded pictures are laid out on A4 pages. Each page is saved into the PDF as a picture, so it looks the same in every viewer; the text in the PDF is not selectable. Up to 300 pages are converted at a time.
Which documents convert best
Letters, reports, assignments and forms with standard fonts and simple tables give the closest result. Advanced layout features — text frames, multiple columns, footnotes, headers and footers, tracked changes and comments — are simplified or left out. For a perfect copy with selectable text, LibreOffice's own File → Export as PDF is the reference.
Common problems and how to fix them
A picture is missing: it may have been linked instead of embedded; in LibreOffice use Edit → Links to External Files → Break Link and save again. Headings look like normal text: apply real Heading styles in LibreOffice. Text looks different: an unusual font was replaced with a standard one.
Good habits
Keep the ODT as your editable version, check the PDF before sending, and use ODT to Word if the receiver needs to edit the document in Microsoft Word.