PDF.js worked in my experience too, but it's the usual problem with a massively complex format like PDFs - getting 80% or 90% is feasible, and PDF.js does that

PDF.js worked in my experience too, but it's the usual problem with a massively complex format like PDFs - getting 80% or 90% is feasible, and PDF.js does that great, but getting to 100% - things like forms, etc. - is very, very hard. It's easier to just use an existing PDF renderer like PDFium that already has the 100%. The Flash thing is separate. Flash is sadly still necessary on the web - most people won't use a browser without Flash, except on mobile. And the pepper version of Flash is more stable than the other. Where the two connect is that if you support enough pepper for pepper-Flash, then you also have the option to use that pepper support to run PDFium.