Right now we have some plugin -> renderer -> browser IPCs. The renderer to browser portion has been converted to Mojo in r461928.

If the PDF plugin switches to Mojo, are there opportunities where we can go plugin -> browser and bypass the renderer altogether?

@thestig: yes, the idea would be that pdfium would just be a node in the frame tree. it would implement an interface from MUS to participate in rendering and getting input events. pdfium would have access to other system services like networking, to avoid going through the renderer. That way there would only be 1 process for showing a full page pdf, as opposed to 3 today. FYI this is how it worked in Mandoline.

However we're a bit far from this, and we do need MUS to ship on desktop first to support this.

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Updating the title of this issue, since the solution may not be S13N.  Talking w/ Justin and John, we discussed a possible option where we pull in PDFium into the renderer process directly (i.e., drop the plugin process and treat it more like a media element) and in the process explicitly remove the dependence on PPAPI.

Timing wise, I'm upping the priority from P3 to P2 to reflect that this is work that will become increasingly important as we approach Dec 2020.