Just a note for anyone interested, with the 2357 branch the internal PDF viewer no longer works.
https://bitbucket.org/chromiumembedded/cef/issues/1565/re-implement-pdf-viewer-using-out-of

You can of course enable NPAPI (which is disabled by default these days) and use Adobe Acrobat Reader

Example of how to enable NPAPI
https://github.com/cefsharp/CefSharp/blob/master/CefSharp.Example/CefExample.cs#L48

This has been fixed in upstream and will be resolved in the 2454 branch aka 45 (which actually removes support for NPAPI, so it's pretty much required).