Actually I believe this has to do with the PDF.js version. After trying several PDF’s it doesn’t necessarily have to do with just pdf’s that are multiple pages, but possibly the dimensions of the page. There are javascript errors when uploading, mentioning invalid canvas size.
When doing a quick search I came across these two articles which both point to updating the pdf.js version.
https://github.com/mozilla/pdf.js/issues/10778
https://stackoverflow.com/questions/55917554/invalid-canvas-size-while-rendering-pdf-to-png-using-pdf-js
