This question comes up constantly, and the honest answer is: it depends on the ATS, but DOCX is the safer default for maximum compatibility, with a well-built text-based PDF as a close second.
Why DOCX is usually safest
Most ATS platforms were originally built around parsing Word documents, since that's what the format looked like for two decades of corporate hiring. DOCX text extraction is generally more reliable across older or smaller ATS vendors.
When PDF works fine
Modern ATS platforms (Workday, Greenhouse, Lever) parse text-based PDFs without issue. The failure case is a PDF that's actually a flattened image — exported from a design tool, or a scanned document — which contains no extractable text at all.
How to check your PDF is text-based
Open the PDF and try to select and copy a sentence. If you can highlight and copy the text normally, it's a real text-based PDF. If nothing selects, it's an image, and no ATS can read it.
The practical recommendation
If a job posting doesn't specify a format, submit a DOCX. If you only have a PDF, make sure it's text-based (most resumes exported from Word or Google Docs are). Avoid PDFs exported from Canva, Figma, or similar design tools unless you've explicitly verified the text is selectable.