DOCX is Word in the OOXML format, with headings, tables and named styles. PDF is a frozen page layout with no heading structure a machine can rely on. LibreOffice sits between the two, and this page says exactly what happens to a DOCX file on the way to becoming a PDF one.
What runs when a DOCX file becomes PDF
LibreOffice 24.2, on a machine we rent and watch. Your DOCX file is uploaded once, LibreOffice runs once, the PDF comes back, and neither file is kept. DOCX to PDF is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the DOCX into the PDF
The DOCX page layout, exactly. LibreOffice renders your DOCX file the way it would print it, so margins, columns, page breaks, headers and footers land in the PDF where the author put them.
Fonts, embedded in the PDF, so your DOCX document reads the same on a machine that has never had those fonts installed.
DOCX tables, drawn as they were drawn. Borders, merged cells and column widths survive into the PDF because they are painted rather than re-described.
Numbered and bulleted lists, footnotes and the DOCX table of contents, all frozen into the PDF at their rendered positions.
What DOCX to PDF costs you
Editability. A PDF is the end of the line: nobody downstream can change a sentence without going back to your DOCX file. Sometimes that is exactly the point, and sometimes it is exactly the problem.
Structure a machine can use. PDF has no heading hierarchy, no list semantics and no table model, so everything the DOCX file knew about itself stops at the print stage. A program reading the PDF sees positioned glyphs and guesses the rest.
Tracked changes and comments print into the PDF according to the DOCX document settings, which is rarely what people expect the first time they try it.
Where DOCX and PDF files come from
DOCX. Word writes it, and so does Google Docs on export. It is ISO/IEC 29500, a zip of XML, which is what lets a program read the document without opening Word.
If a model is going to read the PDF
Worth knowing before the PDF goes anywhere near an assistant: this is the wrong direction for that. A PDF is a set of positioned glyphs, and a model has to reconstruct the reading order before it can begin. If a machine rather than a person is going to read this DOCX file, convert it to Markdown instead and skip the reconstruction entirely.
DOCX to PDF, measured rather than promised
Running DOCX to PDF against the real engine with a real file, LibreOffice wrote 11,917 bytes of PDF in 1.2 seconds, machine otherwise idle. That is one DOCX file on one day and not an average, which is why the number is given together with the file that produced it.
The DOCX to PDF verdict was reached by a format signature, meaning at least four bytes unique to the format were found at the right offset. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the PDF file has anything inside it.
The same DOCX file, sent somewhere else
Other outputs the probe measured out of a DOCX file, so the cost of choosing PDF can be read against something. Sizes do not compare across engines, because each family was probed with its own DOCX witness file.
DOCX to MD: 266 bytes in 0.1 seconds, verified by a pandoc round trip.
DOCX to EPUB: 5,040 bytes in 0.1 seconds, verified by a pandoc round trip.
DOCX to TXT: 264 bytes in 0.1 seconds, verified by reading the text back.
DOCX to HTML: 4,018 bytes in 0.1 seconds, verified by a pandoc round trip.
DOCX to RTF: 4,205 bytes in 1.1 seconds, verified by reading the internal type and finding a witness word.
DOCX to ODT: 12,806 bytes in 1.2 seconds, verified by reading the internal type and finding a witness word.
Other ways into PDF, and what they measured
Among the published routes into PDF, DOCX is the number 20 largest output of the 26 measured. The witness files differ, so this ranks the probe run and not your document.
DOC to PDF: 11,917 bytes in 1.1 seconds.
XLSX to PDF: 18,965 bytes in 1.1 seconds.
XLS to PDF: 18,519 bytes in 1.0 seconds.
PPTX to PDF: 20,929 bytes in 1.2 seconds.
PPT to PDF: 20,740 bytes in 1.1 seconds.
What we will not pretend about DOCX to PDF
A DOCX file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A DOCX to PDF run that passes 90 seconds is killed, and the LibreOffice process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
LibreOffice is serialised on purpose, one document at a time per worker, because each soffice instance takes 150 to 300 MB on a machine with 3.8 GB. Measured ceiling: 23 DOCX conversions in flight and roughly 0.9 a second sustained. Past that the queue answers 429 rather than falling over, which was tested at 32 at once.
One DOCX file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
DOCX to PDF: what people ask
What actually converts my DOCX file to PDF?
LibreOffice does it, 24.2, on a machine we rent and watch. Not a browser trick and not somebody else service: the DOCX file is uploaded once, LibreOffice runs once, the PDF comes back, and neither file is kept afterwards.
How long does DOCX to PDF take?
On the file the probe used, LibreOffice took 1.2 seconds and wrote 11,917 bytes of PDF. That is one real measurement on one real DOCX file, not an average and not a promise about yours: a larger DOCX takes longer, and past 90 seconds the run is stopped.
What do I lose going from DOCX to PDF?
The one to know about first: Editability. A PDF is the end of the line: nobody downstream can change a sentence without going back to your DOCX file. Sometimes that is exactly the point, and sometimes it is exactly the problem.
Is the PDF searchable afterwards?
Yes. The text in your DOCX file is written into the PDF as text, not as a picture of text, so it can be searched and copied. What the PDF does not have is structure: no headings, no list model, no table model. It reads well to a person and parses badly to a program.
Is DOCX to PDF free?
There is a free allowance every month, and one DOCX to PDF conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse PDF.