PSPDFKit for Web is a hybrid renderer. It can render on the client or server, depending on device capability and power. This allows you to view a 500MB+ file even on a slow connection via your smartphone, make notes that are instantly synced to our on-premise server via
https://pspdfkit.com/instant/ and then later you can continue to markup the file on either your desktop browser or maybe a native iOS app such as
https://pdfviewer.io/.
We currently deliver SDKs for all major platforms with Windows coming in Q4 and they all have the same high-fidelity renderer and deliver the same result.
With pdf.js, how the output renders also depends on your browser, and it just doesn't support some of the more complex graphics with knockout groups, patterns and gradients.
See Mozilla's presentation where they say that full support for these is not even planned: https://youtu.be/4yLzRoErOHw?t=20m27s
Our customers are in Enterprise B2B space. Some might still use IE11 (which we fully support). Some care that their PDF files do not leak and only want streamed access. Most care about annotation syncing. Some apply watermarks on-demand. We solve a different category of needs than the more consumer-focussed pdf.js.