What is the size of the JS libraries for the PDF Embed API?

The documentation indicates the PDF Embed API is completely client-based but I don't see it clearly stated whether it's 100% JavaScript (a la Mozilla PDF.js)  or whether it may utilize a PDF engine built-in to mobile or desktop operating system or browser (such as the PDFium-based viewer in Chrome Android). I'm assuming it is pure JS with no client PDF engine dependencies - is this correct? If so is the total download size comparably sized to PDF.js (32MB)?  Are there performance expectations vs. PDF.js?